The Alpha Squad (精鋭班 Seiei-han?) was the elite team of Garrison soldiers assigned to defend Eren Yeager in the last stage of the Battle of Trost District, composed of the best available personnel handpicked by Commander Dot Pixis.
After trainee Armin Arlert proposes a plan for sealing the hole in Trost District's wall, the members of the Alpha Squad are selected in the latter stage of the Battle of Trost District. Chosen are the Garrison's finest soldiers readily available: Rico Brzenska, Mitabi Jarnach, and Ian Dietrich - assigned as the leader of the operation. They and their respective squads are ordered to defend Eren Yeager from Titans, in the hope that he will be able to seal the breach in Trost District's gate. Further details, and the right to cancel the operation, are left to Ian's discretion.
The squad takes the shortest route to the boulder near the gate. Upon transforming, Eren loses control of his Titan, attacks Mikasa Ackerman, and summarily knocks himself out. Believing the mission to have failed, Rico fires off a red signal flare, signifying a serious problem. Spotting this, Anka asks Pixis whether to collect the Alpha Squad but is denied. Rico and Mitabi tell Ian that they should leave Eren and scale the Wall. Mikasa and Ian reject this plan of action, Ian instead giving the order that they and their squads handle the approaching Titans. This begins an argument, with Ian arguing that Eren is irreplaceable, and Rico believing that it is pointless to risk their lives for a "failed human weapon". Eventually, Ian convinces the two that Eren is humanity's best shot, and the Alpha Squad continue as planned.
The Alpha Squad face heavy pressure as an increasing number of Titans enter the gate. Armin Arlert joins with the squad, managing to awaken Eren's Titan and bring it to its senses. Squad Mitabi and Rico suffer heavy losses against the pervasive Titans, but the sight of Eren carrying the boulder spread hope of impending victory. Ian orders all remaining personnel to defend Eren to the death, made more difficult task by the entry of a group of five Titans. Squad Mitabi draw two Titans away, but are killed in the process. Ian is also killed in the latter stage of the operation. Despite a multitude of deaths, Eren manages to successfully seal the gate with the boulder. Thankful that everybody's deaths had meaning, Rico fires off a yellow signal flare to signify the operation's success. Pixis orders the Alpha Squad be rescued, made possible by the Survey Corps and Levi Ackerman's arrival, who kills two Titans closing in on the group.