”Heroic Spirits are anomalies in a sea of aberrations. They are not proof that Humanity is good. They are not evidence that Humanity is sacred. All that they accomplish... is evince the fact that exceptions occur for all laws. Even that of nature itself.”
In spite of their age, Chara is a very mature person, and a very, very empty being. On the surface, they are an unsettlingly computer-like person, speaking softly and often with only the barest trace of emotion in their voice; they use unusually long words, break up sentences into fragments and regard all things with a cold impassivity, in the same way a scientist might behold a new test subject. Their entire personality seems to convey a sense of superiority.
Beneath that mask, they are much the same, if much more complex. Chara is what could be referred to as a disempathic sociopath, capable of empathizing only with specific things; all other things are tools, at best, and utterly worthless at worst. They are cruel, selfish and manipulative; but when something earns their sympathy, they will devote themself to it almost slavishly, providing utmost loyalty and an obsessive protectiveness. Betrayal by something they love, in turn, will be met with fury matching that of the usual Mad Enhancement, turning even the perfectly sane Assassin into a raving beast from Hell. (There is, interestingly enough, no limit to the scope of something they may empathize with; depending on the variant, they still truly love the kingdom they were royalty of, and, regardless of the variant, were at one point willing to die for it.)
They heavily disdain those who abuse their authority or their powers, as well; to believe you are above consequences, as is befitting of someone possessing the exact same mental disorder as Chara themself, is to earn their enmity. They, themself, have to constantly evaluate their every action, consider every single word they speak before saying it, and deliberately distract themself (sometimes painfully) in order to keep their natural impulsivity from rearing its head; to see someone freely indulge themself without expecting anything to go wrong, spitting in the face of all their efforts, will infuriate them. No one is above consequences.
Suffice to say, Chara does not like Gilgamesh.
“I... do not understand why I am also considered a “Heroic Spirit.” I am not a hero. I am a demon. I am not the one who— who freed all Monsters from the mountain. I am not... a good person.”
And yet, in spite of all of this, what they are at their core is broken. They do not view themself as being above anything, even though narcissism is one of the most notable symptoms of sociopathy — their worldview may center around themself, yes, but they easily view anything that defies their remarkably low expectations (that is, anything that possesses basic compassion) as being better than them. Their extensive vocabulary, their constant facade of implicit superiority, even their lust to become more and more powerful — it is all driven by an inferiority complex that matches the ego of the King of Heroes himself in how much it shapes their personality.
Humanity, in their eyes, is evil at its core. Therefore, they, themself, are a horrible person — even when they prove themself to be perfectly capable of empathy, compassion and love. It is what drove them to commit suicide, and it taints their worldview even as a Heroic Spirit. They want to be better— and in so doing, sabotage their pre-existing virtues. They want to be loved— and in so doing, make themself a very hard person to love. They want to protect those they love— and in so doing, become willing to commit senseless murder, simply to become stronger.
They are a very, very difficult Heroic Spirit to be the Master of, and even moreso when it comes to getting along with them; and yet, if their loyalty can be earned, they might well become one of the greatest friends that their Master will ever know.
“...it does not matter. I’ll win the Grail for you, Master. My wish is irrelevant. My desires are irrelevant. For you, my Partner. I will destroy everything in our way. And then, we can both be happy.”