Due to the style of Lovecraft it is hard to determine the exact personality of the Terrible Old Man but he is seen as rather secretive and mostly neutral in regards to others : preferring to be alone, in turn he seems to leave others at peace unless provoked.
When provoked however the Terrible Old Man seems to be capable of incredible wrath, there is also a sinister quality to the character that suggests he frequently keeps contact with strange, unnatural forces (and may very well be one himself).
As with most Lovecraftian monsters the Terrible Old Man may be completely beyond our ability to understand. Making him amoral rather than outright malevolent.
An enigmatic figure by nature, the Terrible Old Man seems to be immortal and has lived longer than anyone can remember : it is implied he can talk with the dead (or otherworldly beings) via his jars and he also has inhuman strength that is described as leaving his victims clawed and mangled to a horrific degree.
Considering the nature of Lovecraftian horror the Terrible Old Man could very well have limitless supernatural power but is beyond human understanding : he is, presumably, far weaker than the Elder Gods / Great Old Ones however.
- The Terrible Old Man's true nature is never truly revealed, leaving readers' to determine what he may be : this is a common factor in Lovecraftian horror and plays on fear of the unknown.
- The Terrible Old Man tale is a sort of twisted morality play in which evil men are punished by dark forces, unusual for Lovecraft (who tended towards a nihilistic view of the world).