The Beast is Kevin's 24th and final identity. It's also volatile and his strongest, deadliest, and most aggressive personality. The Beast first appears at the conclusion of Split, after Dennis is seen leaving flowers for Kevin's late father at a train station. He then boards an empty train car, and The Horde is able to transform — mentally and physically — into what they label "the Beast." When he emerges, he's much more muscular than Kevin typically is; his veins bulge and pulse with a dark liquid running through them, and his eyes turn almost completely black. He's also able to climb on the walls and ceiling like Spider-Man, he can bend steel bars and run at top speed, and he remains strong by consuming human flesh (he eats two girls in Split and chows down on a few others in Glass).
At the end of Split, The Beast is chalked up to being a mixture of the animals that Kevin worked with at the Philadelphia Zoo by police. But in Glass, The Beast tells Elijah Price (aka Mr. Glass) and David Dunn (aka The Overseer) that he came about as the ultimate way of protecting Kevin from his mother's horrors.