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 and had planned their engagement. He mysteriously disappeared while protecting Louise, and his description states that he was eventually captured by the Invaders who saw potential and began experimenting on him as a result. 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 and prefers not to speak about the past. This is probably a result of having guilt for not being able to fulfill the promises they made to each other. He appears to be on good terms with Fedeln Metzelei, being one of the few Invaders who doesn't show annoyance or anger towards him.
Already a skilled combatant while serving in the Regular Army, the experiments performed on him have further improved his strength and agility. He was also given two futuristic tonfas which serve as both melee and ranged weapons. During battle, he gets into such a frenzy up to the point that he forgets everything about his past.
Bersek is briefly mentioned in "Tropical Battles" where he hides a wounded Louise from the enemy before going out to check if the area was safe, against her protests.
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 brings up bad memories. 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.