He is the playboy of the gang. He likes flirting with women and make them blush with his flowery words. He likes women over 18 ( I guess he tries to be legal, no relationship with minors hehe XD) Although he's a playboy he is also the funny guy of the group. Most of the time you'll find him teasing Aya hehe in the end it backfires him (liked the episode when he insisted on opening the flower shop wahahhaa, Yoji was cute and funny there). he is the oldest among the four and although he may not look serious most of the time he acts as the older brother of the four somehow.
Past:
Before he joined Weiss he was a detective. He had a partner named Asuka. They investigated the company Ryotto and during the investigation Yoji was injured and Asuka gave up her life to protect him. Yoji was able to escape. He loved Asuka and couldn't forgive himself. In the following episodes he'll meet a girl that looks like Asuka which is also their enemy. He was reminded again and couldn't harm her although the gal keeps attacking her. Well he really loved Asuka no?