We all know them. Sometimes, we were them. The childhood bully. The person who managed to make some days of second grade feel like...a bad place. I remember my bully; she once tried to feed me a mud pie with goldfish in it, claiming it was soup. I was smart enough to say no (yay!), and she promptly made fun of me.
Ah, elementary school. I so don't miss it.
Anyway, I ran into my bully a few years later, and she apologized for everything. She actually turned out to be a kind girl. Just because somebody WAS a bully doesn't mean that they can't change. I mean, sure, plenty don't. But some do.
Introduce your protagonist's childhood bully. Have they changed? Are they now the love interest? The antagonist? A complete background character? Add a few fragments from your protagonists early childhood. It is always interesting to see where these people end up.