The Gate Guard was in the episode The End - Part 3. He was seen fighting Slade with his axe while guarding a large door. He wears a plain metal helmet without any sort of visor or visual source, (although he does appear to have a humanoid face and at least one eye, as damage to the helmet would reveal) but still shows considerable awareness and skill in fending off intruders despite this. His wicked axe is able to generate fire along the blade, and possesses a separate blade at the other end of the handle. The guard himself seems to be able to withstand average physical damage and extreme heat, and may possibly be in an undead or immortal state. He bears the Mark of Scath on his chest, which is also present on the door he was guarding, possibly indicating some sort of subservience or relation to Trigon or his followers. While he may have had Slade beat in brute strength, a well placed bomb on the door seemed to do away with the Guard while opening the door at the same time, leaving nothing but his helmet and axe. Slade took the axe to aid the Titans in their fight against Trigon.
Theory: A small bit of evidence that the guard was undead is in the fact that while Slade was restored to his full human glory when the door opened, the guard simply vanished, and it is rather unlikely that the bomb would have blown the guard up when lava did nothing to him.
He is the only villain to have never personally encountered the Teen Titans.