Saika is a cursed katana that claims to love all of humanity.
Saika is a demonic weapon that contains a cursed spirit who loves all of humanity. Since it has no body, it possesses the body of whoever wields it and illustrates its love by cutting people. Through this action, Saika can implant a seed of its children into its victims and control them to carry out the bidding of their mother. Saika was said to be in Ikebukuro but disappeared for five years before the start of the series, only recently emerging again in the city.
The truth behind Saika is that it was the blade used to slice Celty Sturlson's soul that connected her head to her body by Shingen Kishitani. When he got back to Japan, he sold the weapon to an antique store owner who was the father of Anri Sonohara. However, after some time, his business began to fail and as a result he began to severely abuse his family. Anri's mother was possessed by Saika and killed her father before she turned the blade on herself. Anri became the owner of Saika and was able to control the blade since she had closed her heart out from love during the beatings her father gave her.
Saika somehow reemerges in Ikebukuro under the command of Haruna Niekawa. She scorns Anri since her lover, Takashi Nasujima, fell in love with Anri instead of her. She is able to command the children of Saika as they go after Shizuo Heiwajima, claiming to be in love with him and want his strength. He is able to put them all off as Anri defeats Haruna.
Anri continues to hold on to the blade when the Yellow Scarves emerge. She uses her blade to ward off attacks made by Horada and the Scarves. She is later rescued by the Dollars as she goes to the final confrontation of the Scarves and Dollars where she is able to use her sword abilities to save Masaomi Kida.