Subject Zeta is an Alpha Series Big Daddy that originates from Rapture, though is first seen aboard the alien spacecraft known as Mothership Zeta.
Zeta, as a Big Daddy, is totally incapable of regular speech. Instead, he emits moans and roars similar to those of whales. Despite this, Kate and her team find out about his past through salvaged audio logs from his home.
According to the logs found, Subject Zeta was first tested with the imprinting process on a Little Sister with mixed results. Apparently he would protect the Little Sister with his life when she is in danger, but when she was not at risk he cared not for her. This led to him wandering away from Rapture and into the deep, dark ocean all alone. How he eventually came to be within an alien UFO is not known, though he was kept in Cryostasis for centuries until being thawed out by a chain reaction from Kate Spero and Samantha Traynor, damaging his pod and allowing him to reawaken.
Zeta is immediately hostile to the Aliens upon being released, making use of his Rivet Gun and Drill to lay waste to his scrawny attackers. His raw strength allows him to ram through cover and -paired with his drill- even lets him destroy windows and open the ship to the vacuum of space. His own strength allows him to hold his grip even against the vacuum, and his suit allows him to survive in space seemingly unhindered.
Despite his simple mind, Zeta does show the capacity to think and solve problems. He immediately seems to know that his Electro Bolt Plasmid can be used on Alien panels to overload connected systems, and he shows fascination as he is struck with awe upon entering space itself with Kate.
While the experiment to bond him with a Little Sister initially was deemed a failure, Zeta seemed adamant to protect Kate, Samantha and especially Tempus upon meeting them. In particular, Zeta seemed to want nothing but Tempus' safety, following her at all times as she called him "Mister Bubbles", though she was never sure why.
Drill - Subject Zeta has access to a Drill that can make short work of enemies at close range. He can use it to Drill Rush, and it never seems to run out of fuel.
Rivet Gun - The Rivet Gun allows Zeta to launch metal rivets at high speeds to impale targets in a similar fashion to the Railway Rifle found in the Wasteland.
Electro Bolt - Zeta can unleash powerful electricity that can chain to and stun multiple targets, as well as disable electronics including drone units for a short time. Can also be used to overload Alien panels.
Incinerate! - At the snap of his fingers, Zeta can start a spark of flame. He can also throw balls of fire to melt targets and start flames.
Telekinesis - Though he relies primarily on his raw strength, Zeta can lift objects with his mind. This can be used to throw back projectiles.
Winter Blast - Turning enemies to ice in an instant, Zeta readies hostiles to be easily shattered and killed with a melee attack.