Orson
Orson is a ghost in the Pac-Man series, serving as a the Luigi to Pac-Man's Mario. He first appeared in Pac-Man (The First One) as a playable character. He is known for his iconic Toc-Man mech, which he uses to explode Pac-Man into gore.
Pac-Man (1980)
Orson appeared in the original Pac-Man as the Luigi to Pac-Man's Mario. He was able to be controlled in the two player mode alongside Pac-Man himself. He was unable to eat pellets, as he was a ghost. Orson is able to collect power pellets, turning him into Small Toc-Man, originally known as Toc-Man because a big version of Toc-Man didn't exist yet.
Orson was well-received by critics, calling him "The Luigi to Pac-Man's Mario" and applauding his green color (due to 1980 being "The Year of the Color Green").
Ms. Pac-Man
Orson did not appear in Ms. Pac-Man because he is not attracted to women. He has gone on record saying "I respect Ms. Pac-Man but I am not attracted to women so I did not help her in the two player mode of her game because I was busy kissing Clyde from Pac-Man in the arcade cabinets for Pac-Man."
Pac-Man Plus
Pac-Man Plus did not feature Orson due to its product placement of Coca-Cola, which Orson has gone on record saying "Tastes good but is Red like Blinky from Pac-Man who tries to kill Pac-Man."
Super Pac-Man
Orson appears in the secret ending of Super Pac-Man. If you beat the game in under 40 minutes, Pac-Man will deliver a birthday cake to Orson.
Pac & Pal
In the manual for Pac & Pal (referred to as "Odd" by critics for having a manual as an Arcade game, which is hard to put in an accessible spot in an arcade) it is explained that Miru (The titular pal) is a friend of Orson that he met at The Gay Bar and introduced to Pac-Man because they have a common interest of Picking Up Fruits (Food).
Professor Pac-Man
One of the questions in Professor Pac-Man asks "Who is the Luigi to Pac-Man's Mario?" and the answer is Orson.
Other Games
Orson appeared in some other games too, where he served as the Luigi to Pac-Man's Mario.