Kretin

Kretin is a molecule with two forms called Alpha Kretin and Beta Kretin who, in Mario & Luigi: Bowser's Inside Story, kidnaps Princess Peach. After a long chase in the Flab Zone, Mario & Luigi finally reached Kretin. Using the spin move, Mario and Luigi free Peach from the grips of it, but immediately get attacked by Kretin.

In battle, Kretin will begin in its Alpha Kretin form. Each individual unit of Kretin can be either red or green, and can only be damaged by the Mario brother of the corresponding color. When one unit receives enough damage, it turns blue and defeated. After all the six parts of Kretin are blue, it will fall apart and become six Beta Kretin. All but one of the units will be fakes in this form, so the vulnerable unit will try to hide by rapidly switching places with the others. If any of the fakes are hit, the real one will make another one the next turn.

The Alpha Kretin's only attack is creating a ball of blue energy that will move through its body. It will then start to glow, turning its arm into a cannon shape. The Alpha Kretin will start to shoot balls of energy towards the bros, red for Mario and green for Luigi. This attack is countered with hammers. In its Beta Kretin form, it will create five duplicates of itself and start to jump around, trying to confuse the player by jumping to a different position. If a fake Beta Kretin is defeated, the real one will just create another copy. Its main attack is surrounding the bros in a hexagon shape. They will all start to charge up electricity. When one of the Kretins jump, that's where the electricity will move on to. This attack can be avoided by jumping. After a long fight, Kretin is defeated.

Kretin X
After facing the original Kretin, the Mario Bros. can face an even stronger version of the boss. The colour scheme is slightly different from the original. This boss is reachable at The Gauntlet in the Challenge Node.

Trivia

 * Alpha Kretin vaguely resembles the logo of AlphaDream, the company that created the Mario & Luigi series.
 * "Kretin" comes from "keratin" and "cretin."
 * In the UK version of the game, the boss is known as Alpha and Beta Keratin, without being a pun on "cretin."