Izuru Kira was born to a family of lesser nobility, his parents having been dead since he was young. While in the Shinigami Academy, Kira befriended Momo Hinamori, Renji Abarai, and Shūhei Hisagi, and they were often seen spending time together. After graduation, Kira, along with Renji and Momo, was sent to the 5th Division, where he stayed until Gin Ichimaru became Captain of the 3rd Division. Kira is loyal to both his friends and his duties, perhaps to a fault. Despite the close friendship he shares with Momo Hinamori, Kira shows that his loyalty knows no boundaries when he defends his captain while she is in a grief-induced rage from her own Captain’s apparent death. Despite this, he calls himself a “monster” afterwards for raising his sword against her.

Kira’s Zanpakutou is Wabisuke. Its Shikai command is “Raise your head.” When activated, Wabisuke’s Shikai bends the blade into a square hook. In this form, Wabisuke doubles the weight of whatever it strikes. The ability affects both inanimate objects and living beings. Though a seemingly pointless ability in sword combat, the effect is cumulative. With a second hit from Wabisuke, the weight will double again and so on, exponentially. Thus, after seven or eight blocked attacks, his opponents are unable to lift their own swords. Once Wabisuke’s effect becomes noticeable, Kira’s opponents often fall to their knees with their heads bent down, as if in apology, thus the name of Kira’s Zanpakutou.

Source: Bleach official character books I-III