Mickey Smith was Rose Tyler's boyfriend and later briefly a companion of the Doctor. He helped defend Earth against both the threats of the Cybermen and the New Dalek Empire. He later married Martha Jones, another former companion of the Doctor.
At first, Mickey was a bumbling coward; His capture by the Autons, and initial trip in the TARDIS, left him greatly shaken. When he met the Doctor again, he displayed much resentment towards him. Already, though, he was braver than he had been; He tried to hold off a Slitheen to give Jackie time to escape. Despite these changes, Mickey still displayed clumsiness, like his pratfall while trying to capture Blon Fel-Fotch Passameer-Day Slitheen. Although still unwilling to travel in the TARDIS, Mickey changed his mind, inviting himself along, unwilling to just be "technical support" (or the Tin dog) any more.
Mickey reached a turning point when he met Ricky and witnessed his death at the hands of the Cybermen. Shown a version of himself that was braver, he rose to the challenge. He played a crucial part in the the Cymbermen's defeat and decided to help rid the alternate Earth of their remnants. When the Cybermen found a way to cross over to the other Earth, Mickey was the first to pursue them. When all of reality was threatened, Mickey again crossed Earths, this time with Jackie in tow. They faced Daleks and Mickey held their creator, Davros, at gunpoint. Choosing to return home, Mickey found love with Martha Jones, and the two became freelance alien hunters. Far from his original cowardly ways, Mickey now actively pursued dangerous aliens.