Shirai is one of Keitaro’s few friends from before he moved into Hinata House(though Keitaro and the other characters have a hard time remembering him and Haitani). He is shorter and pudgier than Haitani(Su calls him a hobbit) and is reasonably good in technology. He helped Su create the background projection device for the summer play.
Haitani and Shirai usually show up to taunt Keitaro about his Tokyo University dream and his poor luck with women. When they find out that Keitaro has so many women interested in him, they become envious.
Epilogue
He and Haitani are the announcers at Keitaro’s wedding.