He should be someone that brings into reader’s attention a central conflict in the world. Like a terrorist type character, who wants to punish the government for their misdeeds. Revenge for a past war. Corruption in politics or religion, or similar.
A highly intelligent person or a stupid wizard who just got his hands on something that gave him more power than he should have? A dangerous villain, someone beyond redemption? If he is killed by the end of his arc, the entire story gets a dark moral tone that suggests characters will be killed when they are beyond redemption.
How did he get his power? Item, accident, learned something, stole it, someone gave it to him?