So Ymir likes to read. He even has a kindle. Why? Because he isn't one of those asshats who think that reading electronic books somehow completely alters the story to a crappier version.
Besides, who doesn't want to be able to read then take a break by screwing around on some apps?
He's generally a pretty quiet person and he doesn't have many friends. He doesn't HATE social activity, he just gets overwhelmed with too much of it. So if he asks for alone time, it's nothing personal- he just needs a recharge.
He's a black mamba, so of course- venomous. He only uses his to hunt though, and even still- he doesn't really need to hunt that often anyways.
The blond parts of his hair is dyed because well he's just a super cool guy like that.