The Biffidus is an enemy that appears in Mario & Luigi: Bowser's Inside Story and Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey. They reside in Bowser's body and appear to act as antibodies, attacking any foreign entities that they encounter. Biffidus appears as a small soldier, with a round body, stubby arms and reddish-pink feet. Their face appears pitch-black, save for a pair of white eyes. Each Biffidus wields a metallic spear and a brown shield.
Shortly after eating the giga-carrot, Bowser comes down with a painful stomachache and demands that Chippy (who is actually Starlow) investigate immediately. Mario, Luigi and Starlow go to the Nerve Cluster to find the source of the pain, and immediately encounter Durmite, who breaks through a wall of bone. As Durmite slowly approaches the group, a trio of Biffidus appear to menace them only to be eaten by Durmite, causing it to grow larger. Mario suggests they have to defeat Durmite in order to relieve Bowser's pain, and the Mario Bros. face it in battle.
After defeating Durmite, Mario and Luigi enter the next room and are ambushed by an army of Biffidus that cling to the ceiling. The Biffidus drill into the ground, causing the platform the Mario Bros. are standing on to come loose and fall into the pit below, where the Mario Bros. encounter Toadbert. Toadbert explains that the Biffidus attacked him as well, speculating that they attack anything foreign they find within Bowser's body.
Every few turns or so, up to three Biffidus appear to assist Durmite during its boss fight. They attack by pointing their spears forward and running at the Mario Bros., who can dodge and counter by jumping. On Durmite's turns, it may eat all the Biffidus and spit their spears at Mario or Luigi, which they must jump over; each spear points towards its target. The Biffidus will stand clear of Durmite while it uses its other attacks; in Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey, they run off-screen for that move's duration.
If any Biffidus are present when Durmite is defeated, they jab their spears in its direction as the animation plays, with the explosion of stars blowing them away.
Mario & Luigi: Bowser's Inside Story / Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey
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