Guardian is the fifth and also a last boss encountered in Episode 2 The facilities. He has very muscular, robust and tall figure, with his head resembling a skull of a lizard. His purpose as the name implies, is guarding a portal to Episode 3, which will take place in another dimension, maybe even hell itself. His right hand is replaced with a strong grenade launcher, which can shoot about 15 grenades in one salve. They either explode immediately, if they touch you directly, or hop around for a while before exploding. Due to a small room where the fight with him takes place, it is quite hard to dodge them all without taking any damage, and you will most likely die in one salve. That means you have to use the teleports on the side of the room. Both of them teleport you to significantly larger environments. He is very tanky, possessing largest ammount of HP from all enemies in the game, with 17000HP, so any other weapon but Riveter is completely useless against him, as it doesn't kill him fast enough.
He is encountered in the final level of Episode 2 The Facilities, and the whole level is dedicated just for him, as he is the only enemy in it.