Vulcan

Vulcan is the name given to a hypothetical planet inside the orbit of Mercury.

In real life
Vulcan is also the name of a hypothetical planet that was said to have existed inside Mercury's orbit. For reference, Mercury's orbit is 88 days, meaning if Vulcan were to be any closer, it would have been destroyed by now.

Appearance
Most accounts of Vulcan indicate they have a body mostly comprised of a jagged and shattered brown head with rivers of lava, indicating a very hot-headed (in literal terms) appearance. Vulcan seems to have the most described appearance out of the current Externian urban legends, with most describing Vulcan as wearing a black hoodie and a varying color of pants (some describe it as brown, some black.) and seems to be barefoot according to most.

Personality
Although Vulcan has the most described appearance, it is equated by the lowest amount of descriptions for Vulcan's personality. The basis seems to be Vulcan having a violent personality, often brutally tearing apart local fauna for fun. This seems to not be listed as part of Vulcan's appearance, as the local fauna's blood would have stained Vulcan heavily by now.

Mystery
The mystery of Vulcan's presence has spread to Mercury, who dislikes the mention of Vulcan. Mercury may or may not have relations with Vulcan. If there are anything to do between Mercury and Vulcan, it's likely bad enough for Mercury to not talk about it.