Atuuda is a catfish golem. It appears as a big floating golem with a zizag-shaped mouth and wing-like fins. It is completely white except for its red tongue and a blue magic symobol on its head. Its long tailed is linked to its owner to allow him to drain vital force.
Atuuda was presumably created by a sorcerer of the chang family or their ancestors. It has been passed from member to member until then. Everytime it was used to give the life force of the previous owner to the new one, a technique that shorten the previous owner's lifespan and basically condemn him. It finally became Zuu Mei Chang's. 
Artificial Exorcists Arc
Atuuda is first seen summoned by Zuu Mei Chang just after his grand-nephew summoned For. It grabs Allen's injured leg and Zu uses it to give Allen his life force and heal him.
Searching for A.W. arc
3 months after the events it is revealed that Zu has performed the tranmission ritual on Howard Link to save his life. Link inadvently summons it when touching his scar before Zu explains him the situation.
Back in the present, Howard Link uses it to heal Allen Walker's body taken over by Nea.
Abilities and Skills
Healing Ability: Atuuda is used by its summoner as a powerful healing agent. Using the user's own energy, Atuuda can heal the wounds of others, though the number of times it can be used is directly correlated to the users bodily condition and, thus, is limited if the user is physically weak. the current user Link summons it by touching his cross-shaped scar on his chest.