Spokesperson!Atticus and Actress!One-One. They are only there for a quick joke in Episode 5 but are very memorable for the sheer absurdity of it.
On a similar note, Mirror!Atticus and Mirror!One-One are a (One-)One-Scene Wonder, due to establishing that Car inhabitant reflections have opposite personalities, which is technically Fanfic Fuel.
"Good Guy" and "Bad Guy" from Tulip's game are really just background decorations, but they stuck with fans, to the point that they are official icons on the show's subreddit.
Randall has gotten a lot of recognition for being a friendly, sentient water puddle who is completely willing to help Tulip catch up with the Cat. Some would've liked to see him as the fourth party member, but that would've been too many characters to juggle. He becomes a recurring character appearing in one of the Train Shorts and an episode in Books 2 and 3.
Nancy only appeared in one episode, and only at the very beginning at that, but fans remember her for her friendly personality and Creepy Cute design. She returns for a small cameo in "The Lucky Cat Car".
Alrick is only really important for the Conductor's backstory, but many fans understand why she was in love with him so much, since he was an Adorkable goofball who reliably stayed by her side throughout their entire lives.
Frank the Bear only appears briefly in Book 3, but many fans love him for being a Moment Killer and a genuine Nice Guy.
The Chandelier Escort from "The Debutante Ball Car" gets a lot of love for his design and voice, despite only appearing briefly.
While Tuba is billed as a primary character for Book 3, she only appears for about 4 episodes before being killed off. Regardless, she's beloved by the fanbase for her design, stoic personality, and being a giant gorilla with tubas strapped to her back.
