To be honest, your question has me scratching my head a little. You've described your character as a person with no qualms about manipulating others, all while putting on a sweet face to the outside world. Whether or not you as the author explicitly state the MC's mental disorder at the end of the book, by including scenes in which she lies, cheats and abuses her way to power you've already committed to showing your reader who the MC is on the inside. The chocolate bar's wrapper might not have a label with ingredients printed on it, but I can still tell there's chocolate inside.

Not explicating her condition is a good thing. It leaves us readers wondering what kind of outrageously callous thing she's going to do next.

As to how you can best communicate her insincerity, one way to do it would be to show what's going on in her mind. (Terrible) example:

"Do you love me?" asked the captain. His eyes a watery haze.
I cradled his face with my hand.
"More than anything in the world," I said, and made plans on where to ditch his soon-to-be corpse. His ship and crew would serve me well.

Best of luck, I like your story's premise.

One way you could go about keeping your main character's mental state hidden is to split the main character and narrator into two people. The narrator could be a relatively minor figure, maybe some henchmen in the pirate crew that the MC finds useful for his skills. At first he only sees the MC for what she is on the outside, but he slowly comes to learn that it's all a facade when he reads her journal, sees her push a fellow pirate into an airlock and space him, etc.

I don't like this technique (the narrator is little more than a camera with legs), but it's not without precedent.

Personally I find this one hard to pull off. I - as a reader - would find this development at the end not satisfying (like the development of Daenerys in the last season of GoT).

The problem is that this can/will break the readers image of the character, but probably not in a good way. He might feel betrayed by the protagonist, just like any character in the story would feel if he was to find out she was lying and just pretending all the time.

Alternatively you still can pull this off with the reader knowing - not from the start, but for most of the time. Start your story with the empathic, lovable character you want her to seem and then throw her into a situation where her true self shows. You create a triangle of knowledge between the reader, the protagonist and the pawn character facing the cruel reality. The pawn, believing the fake good nature of the protagonist, is then killed, just leaving her and the reader and the unsettling feeling following you through the story. Now the reader feels with the protagonist and you create the omnipresent suspense when, where and how this charade will blow up into her face.

And just for the record: It is highly usual for sociopaths to be intelligent and self-disciplined. They just blend into society and afterwards you often hear "I'd never thought she was capable of doing something like that."

It sounds like you don't really know what kind of story you want to be telling.

The story of a ruthless psychopath cutting her way towards the throne, is very different from the story of a sweet girl rising through society as the reader gradually realizes how disquieting and heartless she really is; a story of building horror. Different again is a story which shows us her kindly public face, but lets shadowy coincidences and odd notes build up, until we finally get a big reveal that her kindliness is a facade.

None of these versions is "right" or "wrong"; the sequence of events might be identical. But those are different stories, in different genres -- and those are hardly the only two options you've got.

You need to figure out which story you're interested in. What the central stakes are going to be; what the driving force is; what the reader is meant to find compelling. Is your protagonist going to be someone you're shocked by, or someone you're addicted to? Someone you love, or someone you hate? Someone you always knew was dangerous, but didn't know how much, or somebody who breaks your heart into smithereens? Each of these, fundamentally, is a different story.

Try this: try casting your story into "beginning, middle, end." Just a line or two each; boil it down to its barest bones.
Is it:

Beginning: Protagonist is a nobody;

Middle: she manipulates her way into prestige and power;

End: she takes over the whole universe

Or more like:

Beginning: Protagonist works to make the world better and acheives some initial victories;

Middle: As she gains power, we see she's kind of creepy, and maybe always has been;

End: OK she's been a psychopath all along, angling to be crowned Empress, and she succeeds

Or: Something else! It's your book and your story! Figure out what's the story's core for you. And that will help you figure out what role the protagonist, and her pathology, should be playing.

I will agree with both Anna Fitgerald and Viktor Katzy:

First, as Viktor says, I don't think saving the sociopathy for the end is a good idea, and

Second, as Anna says, if the narrator knows her thoughts, I don't think you can hide it anyway.

Where I differ is that the narrator doesn't have to have an opinion about what they are narrating, they don't have to summarize and call her out as a sociopath. They just describe the important facts and thoughts necessary for the reader to understand what is going on.

So one technique you can use here (and should use very early to establish it) to increase the suspense is surprise. Think of your MC as a predator animal, living very much in the moment. She is an opportunistic killer, her plans and thoughts do not normally include killing. She doesn't have to take any delight at all in killing. It is a high-risk tactic, she could be caught, people fight back, she could be injured or killed herself. Her plans could be exposed, she could be exposed.

High functioning sociopaths don't want to get caught and take pains to not get caught. They treat people as objects, pawns and pieces to be manipulated by various means, often with money. They do understand pleasures, sexual, sensual, drug induced and so on, those pleasures are often their own entertainment, and they deploy them as means of manipulation.

So killing is a last resort, or emergency resort, she would much rather use other means to get her way. Bribery, sex, blackmail, threats, drugs, framing people for crimes, engineering public embarrassment, humiliation and ridicule, faking evidence, arson and explosions, using prostitutes to seduce men under secret surveillance, using hit men or mobsters or gang members to do her dirty work and make it easier to appear innocent.

She is not above pulling the trigger herself, but evil people seldom gain power alone: They have a gang of like-minded loyalists that are brutal, and loyal only because of the money and power they are granted by the queen. It isn't love or sex that keeps them in line, it is avarice, and practicality: None of the henchman try to take the throne, because the first to try will be weakened and slaughtered by the rest; it is a pack of lions (that will attack and eat their own injured).

The advantage of making her kill as a last resort (but we must see this play out early to establish it) is the plans she is thinking about normally avoid killing, but when she makes a mistake and it becomes necessary, she is quick and ruthless. It's a knife in the neck to sever the vocal cords, then she has a big mess to clean up, and too bad because she was looking forward to her date later, and has to cancel it.

The advantage of her having henchman, however she recruits them, is she can give them vague orders that they carry out: "This would be easier if he resigned in scandal. Don't you agree, Charles?"

Sociopaths surround themselves with sociopaths, that hire more sociopaths. They accumulate them in their path through life, finding a way to form a partnership, but she needs to keep an edge over them. That will seldom be sex with the sociopathic henchmen: they also have no romantic feelings about it and are just as satisfied with a prostitute or pickup. So it should be some other kind of advantage the henchman can't buy.

That said, sex can be a very effective tool in manipulating non-sociopaths with useful positions of power into do her bidding.

As others have pointed out, since you are in the main character's head, it's very hard to hide the fact that she feels no empathy. We are in her head, we know what she thinks and feels.

That said, if we agree with the character's goals, their actions might appear understandable, a bit cold but ultimately necessary, etc. At first, that is. You can make quite an interesting experience for the reader, if we start out agreeing with the character, and then gradually realise she's unhinged, we don't want to be her fans. Maybe we even still agree with her goals, but her means and the way she thinks about it all are too much. In that case, you initially hide the character's madness by the very fact that we agree with her, so she's "got" to be good.

If your character is not the main character, so you're not in fact in her head, the task becomes easier. You only show the character's actions, not what motivates those actions.

I have recently read a book, I'm spoiler-tagging the title as everything I say is a major spoiler, and the English translation only came out this last WorldCon.

The Heart of the Circle, by Keren Landsman

The antagonist of this story hides in plain sight, pretending to be the protagonist's friend. In fact, he's a sociopath, manipulating the protagonist and his friends. The novel was written by a medical doctor, so she knew exactly what she was about.

You only know about the character what he says about himself, and what actions of his are observed by the MC. Sometimes his actions seem a bit off: behaviour that disregards social norms because "there will be no consequences, don't worry, it will be fun". Sometimes he's just a bit too calm. Sometimes he makes a weird request. There's always a perfectly reasonable explanation, but those bits of evidence mount up. Until it all hits you in the face.

Even at this point, however, you are not handed a medical diagnosis. You are hit with the realisation that the character in question is a manipulative bastard who has no empathy for anyone and stops at nothing. Mind you, he thinks of himself as "doing what must be done", and his vision of the future is an extremely appealing one, if it weren't for the price.

And that's all that's needed, really. We don't need a medical diagnosis to understand what kind of person that character is. If we do know about the existence of sociopathy and how it manifests, everything makes perfect sense. But that's a bonus, a diagnosis the reader does by himself (or gets the full understanding after reading the acknowledgements, where the author thanks a psychologist for helping her with information on sociopathy, among other things).

I was going to comment but am not yet allowed to so I'll pour it into an answer instead. Let me know if it's not useful so I can remove it if necessary.

You could try to do this by describing the character as a sweet girl (or however you want her to appear) whenever a description is called for, but then when it gets really down to it have her make harsh decision that are backed up by her being or feeling forced to make those decisions.

If you manage to convince the reader that she was in the right or the situation was indeed dire enough for harsh measures they might not view her as the sociopath you'll reveal her to be eventually.

This might even make the reader agree and side with her, also turning the sociopath reveal onto the readers themselves, which could be a cool effect.

The downside of trying to hide the true nature of this character until the end of the book is that it would force you to write and portray a 'fake' personality for her.

This could be picked up by the reader or make the character harder to relate to, causing them to not really care when she gets 'revealed' at the end.

Another effect this could have is when the reader does relate to the character in the way you describe her throughout the story but then see the character is suddenly someone else entirely, changing almost last minute.

Unless pulled of really well or in an original way, this might feel gimmicky just to get a surprise ending or cause the reader to feel betrayed, either by the character or by you as the author.

I like Roger Zelazny's way to accomplish this: his characters are highly competent, very motivated individuals.

He just never mention what kind of monsters they are.

Sure, the reader can infer that they are (often) mentally abnormal, but we're following their story, and they see themselves as highly competent and very motivated individuals, not as sociopaths. Which they often are.

This makes sense, and it adds a layer of subtlety by creating a situation where the reader can realize he's reading about a anti-hero, or can miss that point entirely, and he'll enjoy the novel either way - which is great.

I was pondering this and had a idea based off The Murder of Roger Ackroyd by Agatha Christie. What if this book was a first person narrative? Then, the antihero could describe the events that happened in this story through a biased lens, allowing her to conceal her real actions. This narrative fit in this book's world as an account from her perspective meant to justify herself in the face of criticism from certain people who witnessed her rise. You could delay stating that fact until the very end, with some sort of conclusion where the antihero could call out the people who are criticizing her. I think this would be a great twist that could quickly change reader's views of the narrator and cause them to reconsider the whole book.

A very popular and functional way of writing is "Show, don't Tell" (Google will bring up many authors giving their take on it).

It will likely be a much more satisfying ending for the reader to come to the conclusion of "Wow, what a socipath!", rather that coming to the closing where they are just told that is so, whether or not there is anything in the story that would actually back that up.

Does your story have a character who starts out as a loyal supporter of the protagonist, but later realizes what a monster she is, and then turns against her?

If so, that character is an ideal narrator. The reader will follow the narrators point of view and gradually make the same realization with the same dread.

If you want to foreshadow this, you can make the narrator makes excuses for the protagonist. And feel sorry for the protagonist who has to carry the weight of her "necessary cruelty" while the protagonist herself doesn't seem to be carry any weight at all.

This is quite the tricky question. I am going to assume that the character you describe is your protagonist.

How To

The character you describe is best classified as a "sociopath":

a person with a personality disorder manifesting itself in extreme antisocial attitudes and behavior and a lack of conscience

Rather than the more easily concealed "psychopath":

a person suffering from chronic mental disorder with abnormal or violent social behavior; an unstable and aggressive person.

As such, the only way to hide this information from the reader is to hide her inner monologue (If you had a psychopath, or were writing a story about passing on, you could have an utter disconnect between her perception and the real world). Doing so with her as the narrator, or having a narrator privy to her thoughts, would require her inner monologue to be outright missing in some sections where she is deceiving someone.

If you were to change her from sociopath to psychopath, then you could hide her lying under her own delusions of "doing the right thing," however that is neither as interesting nor what you asked for.

Thus, the best option to hide her insanity is to have another character on the ground serving as the narrator. This character would have to be unfamiliar with your protagonist, so they can't see through her act. If you are really up for the challenge, you could have your narrator for each of your sections, then they can be disposed of in an indirect way by the end of the section (ex: your narrator for the "alien invasion" section could be tripped when fighting in the last battle, or something). Generally, they should be disposed of (either the story moves past them or they die) after, but not immediately after, they narrate your protagonist acting odd (not that they necessarily have to even note that something is odd).

Should You?

If you integrate small hints correctly, this can be a very fun character to experience. This gives your story re-readability, and subtle enough hints will lead to needing multiple re-reads to get everything, just as cool background details and small lines in movies and TV give them re-watchability.

If, however, you have her acting perfectly normal (not not always having that perfect veneer, but either always having it on or maybe saying some really out-of-place things), then it will feel like an incredibly cheap plot twist.

As a result, whether or not you should is entirely a matter of your appraisal of your writing abilities and your personal vision.

For a person to be unfriendly at least at her kind and within the frame of higher enemy (aliens) there must be good reason to excuse.

Example reason: Female is a good choice, i should add autistic genius plus been black. Scientist and mechanic as skills, and the human community rejects her young age theories and technologies mostly by racism. Then aliens came and events flow. If mankind has adopted her theories and techs, things may be different. Then she finds a way at the refugee camp to develop her first something to begin changing things. Pirate ship -> combat ship -> fleet -> gain control of rare resources -> rage war against aliens and conservative humans alike -> winning war, become leader.

When such a hero fights to save people and punish those who oppose the hero raises. The ghosts of the past however also raise, give that hero the other aspect, acting at a not noble way against those she considers responsible, either actually be or not.

The above setup explains the personality and flow of events i believe and makes sense. Why? Because someone who understand better than others can become angry with others blindness especially when that blindness costs himself (life of beloved parents?). But the person must also have the potential to become a great leader. So the combination of existing potential with racist pacification and disaster as a result may fully explain the build of such personality that will, finally, save her kind, but never forgive it because considers them responsible for the disaster.