Yūichi Shibata

was a Human boy whose mother was killed by the Hollow Shrieker when the latter was a living serial killer.

History
According to Shreiker, while he was killing Yūichi's mother on a balcony, Yūichi grabbed his shoelaces and sent him tumbling down. After Shreiker became a Hollow, he sucked out Yūichi's soul and put it in a parakeet.

Agent of the Shinigami arc
This parakeet was given to Chad by a friend named Harutoki who explained that all of its previous owners had died. This, of course, was the outcome of a deal made by Shreiker with Yūichi: that if the latter ran from the former serial killer for three months, he would bring his mother back to life. Rukia explained to Ichigo, Chad and Yūichi that this was impossible, meaning that Shrieker was only lying to make a game out of murder. Ichigo exorcised Yūichi with Konsō and thus sent him to the Soul Society.

Soul Society arc
Shibata appears again in volume 9 and fulfilled an earlier promise with Chad that he would let himself be carried. He joined a surrogate family with Horiuchi Hironari acting as his older brother. He had not yet found his real mother which may be because souls are sent to separate districts in Rukongai, even if they are family. There has been some speculation that Shrieker had eaten the souls of those he killed while he was living, if they were still pluses. Although this is unlikely, it is possible and would explain why Yūichi has not found his mother.

Appearance in Other Media
In the video Game Bleach: Dark Souls, Yūichi is an unlockable character. He appears to be very weak because of his size and poor range, easily making him the weakest in the game.