Sir Gron was a cat who was killed during the crusades during the 13th century in Happy Tree Town. The undead crusader was revived after lightning struck his rotten corpse.
His helmet is rusted (the orange spots), and he is still wearing his tatterd crusader clothes. He has no arm because it was chopped off before he died. Despite being a zombie-like creature now, he is actually very kind to other residents of tree town, well most of them. Petunia hates him because she thinks he looks scary. He also hates Flippy because in episodes with both him and Flippy, Flippy tends to rip off one of his bones and use them to kill other tree friends. Sir Gron doesn't scream when his body parts come off because he is already "dead".
Sir Gron rarely dies (only one confirmed death, aside from two debatable deaths). His injuries and deaths most of the time involve being mutilated (however, his body parts did not dismember when he smashed into a pole). All of his debatable deaths involve explosions.