Fedeln Metzelei is a member of the Metzelei Four, the strongest four of the Invaders and an antagonist in Metal Slug Attack. He is a butterfly-like humanoid creature working for the Invaders that has the power to emit a powerful poison from his wings. However, his poison is also uncontrollable and thus Fedeln is kept sealed off from the other Invaders for their own safety.
Fedeln first appears in "Butterfly Disaster" where he and Nowan are sent on a mission to Earth and get in a battle with the Ptolemaic Army. During the fight, Caroline shoots Fedeln’s wings, somehow causing them to stop secreting poison. Fedeln deliberately enters the line of gunfire hoping to get shot, but Nowan intervenes and pulls Fedeln away, not wanting to be chastised by Annette for allowing a valuable member of their army die. After the two escape back to the Invader forces, Fedeln asks Nowan to go with him to Earth again but Nowan refuses, causing him to call Nowan cruel.
He appears again in "Try Line 2nd" where he and Schwarz accompany the Great Mother into a battle between the Rebel Army and Mars People. When the Great Mother is wounded, Fedeln says that he wants to hug her leadng Schwarz to say she doesn't like him. Fedeln then tries to hug Schwarz, causing her run off.
In "United Front the 8th", Fedeln approaches Schwarz and Franke as Schwarz attacks Franke for a rude remark she made. Fedeln tells Schwarz to be more careful, but accidentally singes Franke's back with his poison. After steadying himself, Fedeln informs the two that the leader has increased the rewards for Operation Beta and wants samples of a golden object located on Earth. Upon hearing this Schwarz, initially reluctant to go with Franke to Earth, changes her mind and heads off. Franke thanks Fedeln for changing Schwarz's mind but after she leaves Fedeln remarks that he has his own ulterior motives for telling them of it.
He makes a minor appearance in "Fall of Disaster" when Schwarz contacts him for a ship, but Fedeln instead whines about being lonely.