Grand Fisher is a hollow who spent over fifty years evading the Soul Society before Ichigo finally brought him down. He's infamous for killing Ichigo's mother, Masaki—though the circumstances differ between versions of the story. The anime portrays him as specifically hunting women, while the manga suggests he targeted children and Masaki was simply in the wrong place.
His hollow form is distinctly odd-looking: imagine a giant hamster crossed with something from a deep-sea nightmare. He's got bird-like limbs and a tentacle dangling from his forehead that works like an anglerfish's lure. This lure is his defining feature—it can transform into anyone he's consumed, and as long as either it or Grand Fisher himself survives, he can't be permanently killed.
In combat, Grand Fisher uses the lure as a psychological weapon rather than bait. He transforms it to look like his opponent's loved ones, counting on them to hesitate rather than attack. He can also harden his hair and claws into spikes for stabbing or restraining enemies. When those spikes pierce someone, they absorb their target's memories, which is how he manages such convincing recreations of his victims. Outside of fighting, the lure works as advertised—a tool for identifying humans with significant spiritual power so he can hunt them.
After losing to Ichigo, Aizen had him converted into an imperfect arrancar by Iceringer and used him as a test subject. In this form, Grand Fisher became far larger and more humanoid, with an unsettling tattooed face emerging from his jaw. He carries a colossal zanpakutō, and when he unsheathes it, he transforms into something even more monstrous with bull-like horns and a skyscraper-sized blade. He returned to the human world seeking revenge on Ichigo, but ended up killed by Isshin instead.
Content compiled by AnimeList.moe from publicly available sources.