Fiyero is the male romantic lead of Wicked and just like Marius part of a love triangle.
Possible Major and Minor Spoilers:
Fiyero is a character that is very hard to get to know. He goes through a very incredible arc through the course of the story. He starts out as a bit of a jerk when he arrives at Shiz. This one moment is important: he starts going out with Glinda.
In the Lion Cub Scene, Fiyero starts to soften up a bit. It almost appears that he has a crush on Elphaba, but deep down does not realize it. The Lion Cub Scene is his first big major character development scene. What is fascinating about Fiyero is that he seems to develop a lot during speaking scenes a lot more than singing scenes. All throughout act I, he has a stronger connection with Elphaba than Glinda foreshadowing act II.
Act II, at the beginning he is hearing all of these rumors about Elphaba. He knows they are not true. This is the moment when he realizes that he loves Elphaba. So he goes from a jerk to someone who cares and now he has matured. To Glinda’s surprise, he choose to break up with her. You find more maturity in him when he confesses his love to Elphaba during “As Long as Your’e Mine”. But you see just how much he loves her when the guards are about to arrest her, but he arrives and tells the guards to let her go. That to me takes courage. So he is also courageous and due to that, he is taken to the fields to be tortured.
Fiyero is much more complex than he appears. He starts out as this cool guy waiting to have a good time who is a bit of a a jerk to this mature and courageous young man. By the end, he has accepted Elphaba for who she is.