Batman: The Animated Series: 10 Best Characters Who Only Appeared In One Episode
3. Charlie Collins - Joker's Favor
While the episode "Joker's Favor" is well-known for being the first ever appearance of the immensely popular Harley Quinn in any Batman media, the episode is also well-known for delivering a great one-time character in Charlie Collins.
Driving home after a bad day, Collins is cut off by a driver who he insults. This driver is revealed to be the Joker, who spares Collins as long as the average man does one favour for him in the future.
Two years later, Joker calls upon his favour and manages to find Charlie, who has moved his family to Springdale, Ohio and even changed his name. He is shocked to learn the Joker had found him and that his favour is simply to hold a door open for Harley Quinn as she pushed in a cake to Commissioner Gordon's award ceremony.
As it turns out, that was a ruse and Charlie is superglued to the door as Joker paralyses the rest of the room and sits a bomb on Gordon's lap. Charlie manages to contact Batman just in time to save everyone, and even happens to get his revenge on the Joker by threatening the clown with a dud bomb, which results in the Joker squealing to the Caped Crusader in terror.
The events of the episode lead Charlie to be thankful that he is just an ordinary guy after the ordeal he suffered, and showcases perfectly how every interaction is important as you do not know whether the guy you yell at who cuts you off is a crazy maniac - which sounds awfully familiar to the plot of Unhinged...