|Hrothbert of Bainbridge|
Bob's original name is Hrothbert of Bainbridge. Sel particularly enjoys Bob as he's snarky, he's dead and he's pretty close to her age. He also has a perpetual case of agita as Harry never listens to his advice. Plus the people that do know about him aren't very friendly to him as they all want something from poor Bob. (Again, MUCH LIKE SELMAK!)
A powerful sorcerer, he lived many hundreds of years ago in England. At some point he authored a grimoire, a book of spells that contained, in Harry Dresden's words, "some pretty nasty stuff," including something called a "Doom Box". Hrothbert fell in love with a woman, another skilled wizard, by the name of Winifred. However, she was tragically killed, and Hrothbert used black, black magic to resurrect her several times. For this crime, he was condemned to be a ghost, tied to his skull for all eternity, unable to interact with the world in any physical way. The story of Hrothbert and Winifred is legendary in some countries, and Bob is still rather sensitive about the subject. (Oh poor Bob!)
When a young Harry came to live with his uncle Justin Morningway, Bob served as the boy's tutor in all things magical, teaching him the principles of black magic, though the practice of same is strictly forbidden by the Council. It is unknown how much Bob was complicit in Justin's actions (murdering Harry's father using a voodoo doll, for instance), but the ghost makes no attempt to stop Dresden or warn Justin when the younger wizard kills his uncle.
In a twisted sort of way, all three are responsible for Morningway's death. Justin attempts to murder Harry by throwing a large shard of wood at him, but Bob calls out a warning. Harry dodges, but in doing so falls on the voodoo doll, inadvertently crushing Justin's heart. Bob's skull is inherited by Dresden along with the rest of the Morningway estate, but Bob apparently is the only inheritance from his uncle that he will acknowledge.
Harry has since made a way for himself in Chicago, straddling the world of the everyday and the supernatural by acting as a private investigator and consultant for the Chicago Police Department on seemingly unexplainable cases. Bob is his secret weapon in this. Not only does Bob contain a wealth of knowledge gleaned from his own wizardly expertise, the ghost has had centuries of experience since being tied to his skull.
Bob is also apparently something of a romantic: he is seen with a handkerchief, wiping his eyes during a movie while a rather bored-looking Harry is watching with his latest girlfriend.
Information on Bob was taken from wikipedia.com.