The phrase "assistant characters" is correct and usable in written English.
It can be used to refer to characters in a story, game, or other narrative that serve a supportive or secondary role to the main character.
Example: "In the novel, the assistant characters play a crucial role in helping the protagonist achieve their goals."
Alternatives: "supporting characters" or "secondary characters".
In The King of Fighters 2000, Rugal appears as a non-playable assistant character (dubbed a "striker") to Kula Diamond.
While not playable, Aerith appears in the fighting game Dissidia 012 Final Fantasy as an assistant character.
The game continues the tradition of Paper Mario, in which Mario can be accompanied by one assistant character at a set time.
In the spin-off title Mega Man Xtreme, he is an assistant character but becomes playable in the sequel, Mega Man Xtreme 2. He is also playable during the prologue and the last chapters from the role-playing video game Mega Man X: Command Mission.
There's two more siblings: Tina Fey, who does not fare terribly well with the crushing pathos here, though acting opposite Timothy Olyphant's lightly brain-damaged sports shop assistant (her character's one who got away) does present quite a challenge.
As the book moves along and a new, impromptu civilization is born beneath the vice president's roof, Ms. Patchett, whose earlier books include "The Patron Saint of Liars" and "The Magician's Assistant," lets her characters' new lives bloom like flowers.
Mr. Garrett becomes the assistant to the character, known as the Most Interesting Man in the World, and wins prizes that include a part in a Dos Equis commercial, $2,500, a tuxedo and three cases of beer a month.
UPDATE: The Advertising column on June 12 in The New York Times and on nytimes.com reported on a reality series on the Mojo HD cable channel sponsored by Dos Equis beer, which sought to find an "assistant" to a character who is featured in the brand's advertising campaign.
They spot Professor Rowan, a Pokémon evolution researcher, and his assistant, the playable character not selected in the game: Lucas (boy) or Dawn (girl).
But perhaps the most obvious smoking gun is the name of Jimmy Smits' murderous Assistant District Attorney character on "Dexter" Season 3: Miguel Prado.
