Flandre.Scarlet

Thanks to her terribly dangerous ability to destroy whatever she gets her hands on, she's forced to live her existence out almost entirely in the basement of the mansion her elder sister owns -- and companionship is rare, the only people she sees is the maid and her older sibling.
Regardless of her seclusion, she adores them both with a little girl's unshakable affection and love.

But this is only one side of the coin, leaving nothing said of the other traits she possesses..

All this time in the cellar has driven her pretty much off the edge in the sanity department. Her kindness is an extremely limited thing, as she only sees the two aforementioned people as friends. Everybody and everything else is as a either a playmate, food, or even more often, both.

Once you see those glowing red eyes and prismatic wings, it's best to run: Flandre's idea of "play" is never gentle, such as tossing flurries of explosive energy at people in a game of "dodgeball" and expecting them to dodge everything.
This is just the tip of the iceberg, as she finds ways to twist up every game you'd expect from a bored child into a life-or-death situation. If her playmates lose whatever game she invites them to or make any wrong step, they will quickly be killed one way or another.

In fact, she finds the pain of her prey to be absolutely hilarious, and if she's bored enough, will painfully torment others in various ways until they die a horrible death.
Often, the last thing a victim will hear before their death is a joyful child's laugh.

Of course, nothing ever goes to waste; not only does she find it a great source of entertainment to messily murder her prey, but it's also a means of a meal, with her hunger for blood and flesh being almost insatiable.
Add all of this to the instinct and intelligence of a natural-born predator, and you have an absolute monster on your hands.

In her eyes, the only people that matter are herself, her sister and the maid. You could call it being self-centered, but she sees it to be perfectly normal, as everybody else will die in the end.
She finds no reason to be intimidated by others, and little causes her to shy away. Being a vampire, sunlight does severe damage to her body, and less things can cause her pain on contact, but these things are few and are naturally avoided.

This inhuman lack of fear, coupled with her sheer amount of destructive power and sadistic idea of amusement means only one thing:

And that would be that it's probably best for the world that she's kept in that cellar.