Bersek is a new character introduced in Metal Slug Attack.
Bersek used to serve in the Regular Army and had a romantic relationship with Louise. They had planned their engagement, but Bersek mysteriously disappeared while trying to protect Louise when she was injured during a battle. Bersek was eventually captured and somehow made his way to the Invaders, who saw his potential and experimented on him. The resulting experiments changed little with his appearance, though a chip was inserted to make him completely loyal to the Invaders.
Though he still thinks of Louise from time to time, he shrugs it off as insignificant, possibly because of the promises he failed to give her. He also appears to be good friends with Fedeln Metzelei.
As a skilled combatant, Bersek carries two futuristic-like tonfas with ease (much like Ariadna) and also has impressive agility despite their weight. In battle, Fedeln points out that he seems to forget everything about his past.
Bersek makes his first appearance in "Memories of Mirage", where he is sent to Earth with Fedeln to find a core from a legendary ship. Being his first time back on Earth since his abduction, Fedeln asks him if they should visit her at least once before they leave; Bersek refuses, as it is something in the past that doesn't mean much to him now. While talking, Bersek brings up the idea of hoping to find metal resistant to Fedeln's poison. They suddenly hear a steam whistle in the distance and find their target; a Mirage Shiee, which Bersek destroys with ease.
Upon retrieving the core, Fedeln asks Bersek if they should get a glimpse of her as they have the time, which he refuses again. They soon hear movement in the distance, prompting Bersek to take Fedeln and hide nearby, even though his poison is likely to injure him. Louise and Reika soon arrive and investigate the wreck, the former realizing it as his combat style. Bersek and Fedeln manage to escape, though he is injured by Fedeln's poison due to being in close proximity with him.