So cuuute

At the last chapter where the main character was looking at the window and was crushing on the other guy.. There was a guy next to him with a pony tail.. This guy is Katou and he was fat in the start.. So that's why when the other guy saw the main character he thought it was his friend Katou. I haven't seen the other manga which based about Katou but you can read it and it's translated just search Ushiro no seki no katou-kun.

Thanks!