THis is just random, but in this manga...
http://www.onemanga.com/Stroke_Material/
the guy's name is Kaname Kiryuu Kun!
Isn't it also a concidence that his crush is a girl called Yumi! That is just soooo close to Yuki...
Crazy theory!!! XD (Of course it's not true...but just think...)
Maybe, this might be Kaname and Zero's child they had together in their yaoi moment, and then Zero didn't know that Kaname used to be a woman (because no one knows whether the ancester of Kuran was a man or woman.) but when Rido brought Kaname back, he put her in Yuki's older's sibling body(which could still be a boy or girl everyone!) and then---
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