Well rebirth is pretty straight forward; once you're lvl 100 minimum you can get reborn, when you have done the quest/fight you can choose your class (Killer, Knight, Warrior, Wit, Seer, Priest), and you will lose 100 levels and get one lvl free :) (you will still keep your stats) just making it easier to lvl again and recieving a cape with stats according to class.

Stats on Capes;
Killer - Atk
Knight - Spd
Warrior - Def
Wit - Matk
Seer - Spd
Priest - mdef

Depending whether you have more str or int will depend what you can choose (str - killer,warrior,knight)
(int - wit,seer,priest)

Hope that helps :) x