The Gelth ambassador was the leader of the Gelth.
Though originally humanoid, the ambassador and billions of Gelth were turned into gaseous lifeforms in the Last Great Time War. Eventually, they discovered a space-time rift, which opened up into Gabriel Sneed's mortuary in 1869 Cardiff. The Gelth were naturally attracted to the area, since they could inhabit the gas found in the pipes and emerge through gas lamps. They also discovered that they could survive in the gases in a decomposing human and, as the Ninth Doctor put it, "use it as a vehicle". The Gelth ambassador contacted Gwyneth, Sneed's servant, who had psychic abilities. She thought of the Gelth as angels.
Through Gwyneth, the ambassador spoke with the Doctor, presenting its kind as peaceful and facing extinction.The Gelth asked for the Doctor's help in allowing Gwyneth to open the time rift further, allowing through more of their kind. The Doctor accepted the idea as an interim stage before taking the Gelth to a place where he could help them acquire new, specially-created bodies. To open the rift efficiently, Gwyneth would have to stand in the weakest point of the rift (under a door frame in this instance) where the strongest connection to her 'angels' could be made. With the rift opened wider, the Gelth, however, planned to let all the billions of Gelth through and take the planet by force. Gwyneth closed the rift in time, however, and destroyed the ambassador and the remaining Gelth with fire. (TV: The Unquiet Dead)