Zuzu's owner

An unnamed individual lived in the Edifice. He was tall, thin and humanoid, with pewter-coloured skin. He wore florescent coloured clothing which seemed oddly tailored to Donna.

Zuzu's owner lived on Floor 165 of the Edifice, apparently by choice. He took his pet Zuzu for a walk everyday, going down several floors. He encountered the Tenth Doctor and his companion Donna Noble on one of these walks. (NSA: The Forever Trap)