How do I make a NPC that will hunt the player down? Also is it possible to make the NPC change their appearance when hunting the player?

If you want someone to stalk your player, just give them a 'find' package, and set the reference to the player. But what do you mean by 'change their appearance'?

by change appearance I mean that every time the bounty hunter encounters the player then they will change their race, appearance, gender, etc.

Well, then you should probably create multiple bounty hunters, each looking different from the others, and give them the same AI, weapons, dialogue, etc.