Koto

Koto was a giant, blue, crystalline bounty hunter who had the special ability of absorbing air, heat and light into his body. He entered the TARDIS whilst it was still in flight after the Tenth Doctor unintentionally carried an escapee from a prison that he had landed on. The bounty hunter intended to absorb the Heart of the TARDIS to kill the Doctor and the escaped prisoner. (DWBIT: Cold Assassin)