Sabikui Bisco, Vol. 9 (light novel) — Shinji Cobkubo

Bisco and Milo are basking in the joys of family life when the strongest mushroom keeper from a parallel world falls like a shooting star right on top of them—and her name is...Bisco Akaboshi! In her timeline, she lost to the rust and just barely managed to escape to the world we know. Bisco and Milo (but mostly Milo) find themselves unable to abandon her and head off on an adventure that will take them across time and space...that is, if she doesn't succeed in absorbing this world's Bisco first!