Subject Psi (Ψ) was the last Alpha Series officially produced by Dr. Alexander, before he and his fellow lab aides abandoned the project. There may have been a Subject Omega sooner if Psi hadn't retained his free will, along with his fury at Ryan for kidnapping his daughter, Aimi Redfearn. The Gatherer-Protector pair escaped Fontain Futuristics shortly before the Kashmir bombings. Aimi's mother was in the restaurant at this point, and was gunned down by Atlas' troops right in front of the Little Sister. This alone caused the furious Big Daddy to go on a solo campaign, venting his formidable ire against anyone and everyone who sided with either Atlas or Ryan.
A decade later, he still patrols the area, protecting the Little Sisters with his now-grown daughter. Psi has grown resistant to Plasmids, after fighting his long war against the Splicers. The Tundrideons see him as the embodiment of anger in all its shapes and forms.
Befitting for a battle-worn veteran, Subject Psi's sleeve armour has been torn off, along with other parts of his suit. Most of the armour has been blackened, due to repeated brawls with the Fire Wardens.
Psi's choice towards Harvesters/Saviors
Subject Psi will act differently towards the player, depending on whether or not they have harvested any Little Sisters.
Sister Savior: Psi will accompany the player throughout the rest of the game, and will continue to do so until the player harvests a little Sister.
At least one Sister harvested: Psi will attack immediately upon seeing the player. This will also happen if Psi accompanies the player (as above) and a Sister is harvested then.
As long as the rouge Daddy is on the player's side, he is invincible to all attacks. Once hostile, he can be harmed by anyone and anything. He has twice the amount of health as Subject Omega. Friendly fire will not provoke Psi's rage.
Psi's daughter has gathered ADAM over the course of her life. Some of this was for the express purpose for keeping her father alive. Along with a Machine Gun, Subject Psi has the following:
Plague - Acid Grenades
I'll finish the rest off later. For now, opinions, please.