White

White is the Hollow Aizen built to test Hollowfication, and it is the origin point for almost everything Ichigo Kurosaki is. It is a near-black body with a white mask that anyone who has watched the series will recognise immediately, blades in place of forearms, and a red Hollow hole in the chest.

It was made from the layered souls of several Shinigami, in the same way an Asauchi is, which gives it the ability the experiment was really about: White can enter a person's soul and fuse with whatever latent Shinigami power is in there, effectively becoming them. It was released into the Human World to find Quincy, because Aizen wanted to know what would happen when the two were combined in one body.

It found Masaki Kurosaki. Isshin Shiba fought it and could not finish it, and White destroyed itself by biting her, which was not a defeat but the final stage of the transfer. It was inside her from that moment, and it passed into Ichigo before he was born. The Hollow Ichigo spends the whole series fighting to control was never an infection he picked up. It was there first.