Iwaizumi was the Beast Hashira. He was one of the strongest Hashira's of the 12.

You, on the other hand, were a demon slayer. After a demon attacked your home, murdering your family, only leaving your sister a demon, you had decided to take avenge and become a demon slayer. After going on a mission with the sound hashira and healing after taking down an upper demon, you were sent to train for a certain amount of time for each Hashira. Iwaizumi is your next mentor.