The goal of the story is for you to get to know your character better, so whatever you choose, it should be either a significant event that shaped his life, or a snippet of life that shows off his personality.

That said, it's also a story. Think of it like a mini-NaNo and apply the same rules:
- stories character, setting and plot
- plots have beginning, rising action, climax, resolution
- your character should experience some change as a result of what happens

Cheers,
Michelle