Tris and Four have always hated each other ever since he cut off her pony tail...in the fifth grade. Their hatred could be called hatred but they can't even pin point it.
Tris is tough and bullheaded. But behind that she's broken and bruised. She had a childhood that people have to recover from. Hell, some never do. Her mind is a dangerous place.
Tobias, Four, is just as damaged. He hides behind harsh looks and sports. People view him as a man whore since he can talk up girls like no other, but that a misunderstanding. He's never acted on their wants. He's broken.
Their minds are endless, dangerous, broken, train tracks.