Naughty Eris and Nice Eris are two characters originally planned to appear in Underfist Versus the Dinosaurs and in the Underfist series. Both are the split personality reincarnations of Eris she's reborn as after her heart breaks in two sometime after Hoss Delgado breaks up with her in the episode Chaos Theory.
After Hoss Delgado broke up with Eris, she did not handle the breakup too well. Eris started to think why Hoss broke up with her, wondering is she's not good enough for Hoss or she's "too good" for Hoss. Finally, her broken heart split in two and so did Eris, transforming her into two new incarnations of herself, Nice Eris and Naughty Eris.
Initially, Both Erises are would appear as the main villains in Underfist Versus the Dinosaurs and as recurring villains in the Underfist series, as they seek revenge on Hoss and to win back his love in their own ways as well as causing trouble and chaos among the other members of Underfist.
Nice Eris is the good part of Eris. She has blond hair that's tied into a bun, wears a white Greek dress, light blue earrings, gold bracelets, and sandals, make her the most similar to the original Eris. She's tries to be the perfect girlfriend towards Hoss, but she can be a nuisance and can be crazily lovesick, since she only wants Hoss to love her. She is the embodiment of "smothering with love".
Naughty Eris is the evil part of Eris. She has long raven hair, wears a dark blue Greek dress, a golden laurel wreath on her head, black bracelets, and black shoes. She is a bad girl who is disrespectful to authority figures, likes to listen to loud music and staying up all night eating ice cream and playing video games. Like the original Eris, she loves to cause chaos and wants Hoss to join in on the fun. But a little bit of Naughty Eris's fun can go a long way.
Due to Maxwell Atoms' contract for Cartoon Network expiring and plans for the series have been cancelled, both Naughty Eris and Nice Eris never came into fruition, but they both make an appearance as cameos during the end credits of Underfist: Halloween Bash.
- Both Naughty Eris and Nice Eris are based on the representation of the two versions of Eris in the Greek Mythology "Hesiod Work and Days"