Michael definitely does not apply to this list. Michael's opening monologue is entirely back story.
This is a hard question because its not clear what is meant by backstory. Does backstory include
Tommy and
Kelly arguing about leaving his sister behind? That's backstory.
Angel doesn't tell us his backstory, but we learned about it from
scratch. Does that count?
Pegs dad was a pilot and taught her to fly. That's backstory.
Puck told us why he's there and where he came from.
So my answer is none of the listed
characters qualifies.
Hell even
Todd has a backstory.