The three enigmatic characters, H-Man, J-Man and I-Man are collectively known as Letter people as their names are preceded by a letter.
All of the Letter people share the same face and pasty complexion. They all wear suits and they all carry briefcases.
H-man wears a gray suit with a blue tie. He wears his hair in a crew cut and carries a black briefcase.
When speaking, he over emphasises the letter s and his sentences are choppy and broken.
I-Man wears a black suit with a black tie. He is bald and carries a silver briefcase.
His speach is monotone and his sentenses are also choppy and broken.
J-Man wears a tan suit with a bow tie. His hair is styled and he carries a brown briefcase.
His speach is normal.
Other letter people could exist. Theoretically, they would each have a different suit and hair styles and briefcase colors.
Their abilities are mysterious and largely unknown.
They have demonstrated the ability to teleport characters at will.
Additionally, the three of them together were able to threaten Xenomancer Red, although it is implied that just one of them could have been defeated by the Xenomancer.
The original ispiration was the enigmatic G-Man of the Half-Life series. He seems to serve as a liason between Gordon Freeman and some unknown employers. H-Man shares several attributes with G-Man, hence the origin of his name.
J-Man was inpired by the Observer(s) of the television series Fringe. The observer appears at least once in every single episode. Inspired by this, J-Man was randomly placed in some of the episodes as a background character, observing significant events.
The collective organization of the letter people was inspired by the Fringe episode "August", in which it is revealed that there are in fact more than one Observer.
Some of their powers and associated graphics are partially inspired by Q, of Star Trek: TNG