Serizawa is the Lead Commander (筆頭局長) and the highest authority in the Shinsengumi.
Although he has a sharp mind, he's quick-tempered, arrogant, and self-centered.
His outrageous behavior is the reason why Shinsengumi became notorious in Kyoto.
He looks down on Kondou and Hijikata.

Source: http://tokio-fujita.livejournal.com/1142.html