When Doctor Who returned to television in 2005, Billie Piper was cast as new companion Rose Tyler. Introduced as a 19-year-old shop worker in the revival series' premiere episode, "Rose," Piper's character first accompanied Christopher Eccleston's Ninth Doctor in the TARDIS. At the end of Season 1, Rose watched the Doctor regenerate, marking the debut of David Tennant's Tenth Doctor. Rose would only grow closer with the Doctor in his new incarnation and the two would travel together for one more season, until they were torn apart in a moment of interdimensional tragedy, trapping them in separate universes.
Source：https://www.cbr.com/why-billie-piper-left-doctor-who/
