Now cut off from the Time Lord race, the Doctor found himself alone and bereaved. Emotionally haunted by the war, he entered a darker period of his life spattered by resentment, where he tended to brood and act rudely to those who rubbed him the wrong way, and found a particular rage towards the Daleks he believed to be destroyed. Despite this, he remained jovial and friendly towards anyone he took a liking to, slowly reasserting himself as the Doctor again while burying the memories of his war incarnation.
Rose Tyler was his most constant companion, although, during the Slitheen family's attempt to destroy the Earth for profit, he also gained a temporary ally in Harriet Jones. The crisis also began an on-going love/hate relationship between the Doctor and Rose's mother, Jackie, as well as with Rose's ex-boyfriend, Mickey Smith.
He also had a short spell of adventures with Adam Mitchell, an employee of Henry van Statten, at Rose's request, but evicted him for almost dramatically altering human history. Adam later returned to exact revenge on the Doctor for booting him from the TARDIS, but redeemed himself as a companion by sacrificing himself to defeat the Master.
After resuming his adventures with just Rose, the pair were accidentally scammed by the ex-Time Agent Captain Jack Harkness. After helping him resolve the Empty Child plague, they welcomed him on board the TARDIS in their travels. There they began a quest to recover Jack's memories that were wiped by the Time Agency, running into a future Mickey Smith, sharing an adventure with the Brigadier, and ending up with a stowaway companion in Tara Mishra.
The Ninth Doctor met his end upon discovering that Daleks that had survived the Time War had been rebuilding their ranks by manipulating the Fourth Great and Bountiful Human Empire to their advantage. Rose briefly absorbed the power of the time vortex and became the god-like Bad Wolf entity. Once Rose destroyed the Daleks, the Doctor absorbed the energy out of her, forcing him to regenerate into a new body in order to survive.