Kretin

Keratin is a molecule with two forms called Alpha Keratin and Beta Keratin 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 Keratin.

In battle, Keratin will begin in its Alpha Keratin form. Each individual unit of Keratin 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 Keratin are blue, it will fall apart and become six Beta Keratin. 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 Keratin'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 Keratin 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 Keratin 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 Keratin 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, Keratin is defeated.

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

Trivia

 * Alpha Keratin slightly resembles the AlphaDream logo that appears while turning on some Mario games (including this game, AlphaDream is a gaming company in partnership with Nintendo).
 * In the US version of the game, the boss is known as Alpha and Beta "Keratin, which derives from "carotene"