The Demi-Hollow

The first appeared as a  that was chained to an abandoned Karakura Town hospital. He is turned into a Hollow thanks to the interference of Don Kanonji and later purified by Ichigo.

Appearance
While still in his Jibakurai form, the Demi-Hollow looked like a man in his early to middle thirties with short spiky dark hair and sideburns, wearing a long sleeved T-shirt. After his transformation he turns into a medium sized bipedal Hollow with a mask resembling the face of a frog and two long extensions coming out the sides of its mask, resembling pigtails.

History
Before his death, the Demi-Hollow was the older son and heir of a rich hospital owner. He had planned to live the good life once he inherited the hospital and started raking in money from its profits. When his father died he left the hospital to his younger brother instead, which caused his spirit to be bound to the hospital after death.

Agent of the Shinigami arc
When the famous medium Don Kanonji, host of Japan's best known ghost hunting show, "Ghost Bust," announces he intends to do a live special from Karakura Town, the whole town gathers in front of the abandoned hospital where the show is going to take place. When the crew begins to trespass into the Demi-Hollow's territory, he starts howling out in anger to try to drive the Humans away. Since the howling and roaring doesn't have the desired effect, the Demi-Hollow comes out of the hospital and starts shouting at the crowd, telling them to back off of his territory. However, the Humans are undeterred, as most of them cannot see or hear him.

The situation changes when Don Kanonji enters the picture. He fearlessly approaches the Demi-Hollow and inspects him closely. The Demi-Hollow tries to intimidate Kanonji, but the experienced ghost hunter is clearly unfazed. he picks up his "Super Spirit Stick" and shoves it into the Demi-Hollow's half opened Hollow hole, hastening his Hollow transformation and causing him excruciating pain. Kanonji tries to calm the suffering Demi-Hollow, saying the pain will soon pass while twisting the stick inside the hole. He promises to free the Demi-Hollow from what he calls the chain of nightmares, for which he uses a gibberish incantation, intended to finish the job with a single blow. After a few seconds the chains tying the Demi-Hollow to the building begin to snap.

When Ichigo manages to reach Kanonji and the Demi-Hollow and push Kanonji away he finds that he's come too late, as the Demi-Hollow's plus form disintegrates before their eyes. A couple of minutes later the Demi-Hollow recomposes himself as a proper Hollow right above Ichigo and Kanonji. He instantly begins feeling the hunger of the Hollows and lunges from the top of the building onto his two opponents. Kanonji pushes Ichigo back, telling him to run so he can face the Hollow himself, and then stands in the Hollow's path armed with nothing except his Spirit Stick. The Hollow attacks but is blocked by Ichigo, who then pushes him back, away from Kanonji. The Hollow seizes the chance, while Ichigo and Kanonji are arguing, to try a sneak attack on Ichigo, but the substitute Shinigami catches on and blocks him.

Trivia

 * The Demi-Hollow is dubbed as such in the SOULs databook, even though he becomes a full Hollow shortly after his debut and retains this form for the remainder of his story.
 * The two extensions on his mask were omitted in the anime.