As the novel opens, Charlie is still reeling from his best (and probably only) friend Michael's suicide. We never really get to know Charlie and Michael as a duo—all we know is that they went to a few football games and peeped inside their neighbors' windows. Seems like tame high school fun, but maybe Michael was window shopping for a new family.
We're pretty sure Michael exists to show Charlie how not to be. Even though Charlie eventually stops writing about Michael, the suicide still haunts him. When he experiences his own suicidal thoughts, he pleads with the reader, "I never wanted to. You have to believe me" (2.15.12), as if to say, I won't end up like Michael. I won't. Maybe he's trying hard to convince himself of that, too.
Michael is the only character in the book who gets a last name. What's up with that?