The Black Knight is one of the most interesting characters in Fire Emblem as a whole. The game teases you by revealing little to no information about him until endgame of the second game!
All of his motives, his pain, and the fact that he makes you shiver every time he appears is only part of what makes him so interesting. HIs desire to test Ike. His honor and ability to acknowledge that Ike is stronger than him.
He is a much more developed character than Ike ever will be.
Ike is strong and he cares about his mercenary family. He wants to do what his father would do. Aside from that, there is nothing about Ike that really makes him all that interesting.