Kaiwan

is one of the four guardians that guard the entrance to the Seireitei. He is the guardian of the Blue Stream Gate (Eastern Gate). He appeared in the manga only.

Soul Society arc
Aizen hypnotized him and two other gate guardians & used them to cause trouble for Ichigo and his friends. But he was stopped by Jidanbō Ikkanzaka and Kūkaku Shiba.