Shrieker

Shrieker (シュリーカー, Shurīkā) is a Hollow with features resembling a frog and retractable bat wings on his arms. He is the fourth Hollow to be defeated by Ichigo Kurosaki, and the second to reveal his history. Shrieker is a Hollow with high intelligence. Like many other Hollows, he is driven to kill; however, he also reveals that he was a murderer before he died. He is extremely manipulative, as he tells Yuuichi Shibata that he would revive his mother if he can run away from him for 3 months, just for the pleasure of killing anyone who gets involved with him. He also uses underhanded tactics in battle, as he threatens to blow up Yuuichi's cage to prevent Chad from giving chase while he hunts down Rukia.

About 4 or 5 years ago before the main storyline, Shrieker was once a serial killer, that killed 7 or 8 people. He roamed around various places killing people using what looked like a butcher knife. Yuuichi Shibata's mother was his last victim. He stabbed her multiple times while she protected her son, eventually following her out to the veranda. As he was about to stab her for the last time Yuuichi grabbed onto his shoelaces, causing him lose his balance and subsequently fall out of a high story window. This event led to Shrieker's death as a Human, but also his inevitable transformation into a Hollow, still holding a grudge against Yuuichi for killing him.

Shrieker decided to play a game with Yuuichi, taking his soul out of his body, and placing it inside a parakeet. He told the boy that if he could run away from the Hollow in his current form for 3 months, he would restore his mother to life. Shrieker only started the game to gain more victims as no Hollow or other creature can restore life to someone who has died. Attempting to save the boy's soul, two Shinigami were already eaten by Shrieker. (Source: Bleach Wikia)