Actually Gary was the first person they showed.
But regardless, Serena's existence seems to be a retcon of some sort. That or, like you and I assume...it was a small incident and they're not childhood friends.
Oops! Scratch that point out then hehe. But regardless, it still would've been confusing to the viewers. Showing Serena before she is introduced is not a good idea, especially if she is a retconned character who was never mentioned before. 1st you introduce, then you explain/show.
Yeah, I don't think they were childhood friends per say, but rather a small (yet REALLY significant) moment.