Watashi ni XX Shinasai/WMG

The guy from the Plain Love one-shot is really a younger Shigure.
So first of all: Kagano from the one-shot, who is supposedly plain, is definitely a Hollywood Homely. If you took his glasses off, he'd look like Shigure with less kempt hair. Shigure's current personality being different from Kagano's can be explained by his relationship with Wakana. Remember how Shigure makes slightly offhand remarks that his perpetually happy face is his facade? He broke up with Wakana because she did something that rendered their romantic relationship irreparable, which in turn is because she is not as stupid as she seems to be. His mother divorced his father and remarried. His stepfather's job as a hospital director causes Shigure to transfer schools and his personality changes into the personality we know in the series (because he made some vow never to be fooled like that again or something). He also made a promise to deal this retribution to the girls in his new class. He ditched the glasses for contacts and became who he is today.
 * Similar character designs from the same mangaka?
 * Eerily similar.
 * The character designs are just similar. Even Hinata from Ema Toyama's previous work Koko ni Iru yo! also resembles Shigure.
 * Sure, but this is a WMG. Also, I'm not basing my theory entirely on his appearance - it's just one facet. I do give more justification than simply "I think they look the same!".
 * Really doesn't sound like that, since the only connection that's brought up is their similar looks. But I like that theory.
 * It's the only connection they have in canon - making the other connections is my job in a WMG, is it not? :) Also, a better reason for the theory being based on character designs would be that I accounted for the differences between the characters' lives and that my theory was not just, "They look the same. Therefore, they are each other."

This is really a shonen series
It would explain all the female ecchi.