User blog comment:ShonenChicoBoy/Shonen's Regimen and Tonic: July 2015/@comment-1493741-20150718164145/@comment-18812574-20150718213522

Turns out it's Uryuu. That's right, Ishiin despite being a doctor (think he's "doctored" maybe what, once in this whole series? Lol) was clueless/never found out, and Masaki just gave the kid she didn't want to Ryuken.

Masaki: "Here you go. This one looks like you anyway."

Ryuken: "..."