Bonea

Harry Dresden's parasite is first mentioned in Ghost Story, and first appears in Skin Game.

Description
In itself, the spirit looks like a small cloud of greenish sparks inhabiting a wooden skull Dresden originally crafted for Bob.

In Dresden's mental world, the spirit looks like a little girl about twelve years old, around puberty. Her appearance is based on Dresden's own as well as on that of many women important to him: she has Dresden's dark hair, Lash's blue-green eyes, Karrin Murphy's square, balanced chin, the Archive's rounded cheeks, Susan Rodriguez's jawline, Elaine Mallory's nose, and Kim Delaney' hair.

Biography
The parasite is a spirit of intellect, born of Dresden and Lash, who sacrificed herself for him in White Night. A part of her remained in Dresden's mind, letting him to play the guitar, and hear echoes of hear in his head. The "parasite" formed from these remains and parts of Dresden's own psyche.

Ghost Story
In Ghost Story, Demonreach mentions the parasite as keeping Dresden alive and metabolically active.

Skin Game
In Skin Game, Mab informs Harry Dresden that he only has three days before the parasite kills him and that he will have to do her bidding if he wants to be cured. The threat stems from the parasite's growth, which at first causes increasingly bad headaches but ultimately threatens to rip Dresden's head open, as the spirit needs to escape it to keep growing. In return for his service, Mab provides Dresden with a magical earring that freezes the spirit, preventing it from growing and disrupting his work. After the earring's magic expires, Dresden collapses from headache, but is saved by the timely arrival of Molly Carpenter, who delivers the spirit and transfers it in the wooden skull Dresden made for Bob.

Word of Jim
In a Reddit AMA, Jim Butcher has officially confirmed her name is Bonea, nicknamed Bonnie, an old Scottish name meaning "beautiful" and a wordplay on "bones".