[Tusjecht]

The side story from Volume 12, Sky Blue Wings, was released in full yesterday. No links here as per AS rules; go Google it.

Spoiler for tl;dr:

Fuuko's legs are biosynthetic prosthetics; the artificial skin has grown upwards and fused with her real skin tissue, so it's not easy to remove the prosthetic.

Her lack of legs was a genetic disability, detected since birth.

Fanservice: her legs seem to be an erogenous zone. I assume we all know what it means when a girl says it's "itchy"...