A completely theoretical take on an Undertale exclusive hardmode boss against the little shit Chara. (Who probably isn't as evil as everyone claims but was enough of a douchebag as they went on to eventually become the evil demon you see at the end of the genocide run, so this might be fun)
The concept of 'who the player' aka: You, is rather strained by the existence of Chara as a past story element. There are some who believe you ARE the first fallen human and some who think the idea runs a bit deeper. They're definately a reflection of your actions, but at the end of the true pacifist ending, Asriel goes on to explain that (you/chara) really weren't all that great which creates the schism of associating yourself, when really you just did a whole bunch of good. So who is the first fallen human if not you?
Anyway, moving off that obvious think-bait.
The horns and the crown of fire is to symbolize that they have all the monster souls and Asriel was the prince of the monsters so being his brother I assume Chara might have desired that power themselves.