Grand Fisher is a hollow who has evaded shinigami for 54 years, and is responsible for Ichigo Kurosaki's mother, Masaki Kurosaki's death. In the anime, he has a taste for women, but in the manga, Masaki was simply in the way of his next meal: Ichigo. Later on he finds out Grand Fisher killed his mother and vows revenge.
In hollow form, Grand Fisher looks like a giant hampster with bird like hands and feet. He also has a lure on his head, like an angler fish. The lure can change its form, usually into a soul it has devoured, and hide his spirit energy into the lures current form. That is how he avoided discovery by the shinigami for over 50 years. The lure can act independantly, and if either survives it cannot be 'purified'. Apart from battle, he uses the lure to seek out humans with high spiritual energy to target. Grand Fisher fights in hollow form by using the lure to keep enemies away,
by turning into someone that is precious to the person. His hair and claws can be extended, to attack enemies, which as daft as it sounds, can be fatal. Once the enemy is stabbed, Grand Fisher can create a lure decoy of him/her to use in later fights.