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Undertail, being the R34 label, means the characters have varying levels of sexualization. The events are always complemented with adult and sexual themes. Sometimes the stories have crossovers between AUs.
Although having sexual themes, Undertail also focuses on a variable amount of couples in the stories, that are made by fans.
Most of the Undertale and Undertale AU characters remain physically the same, but they may have the main personalities and behaviors changed, in order to look more sensual and appealing for the viewer.
- The skeletons may have a magic tongue, genitals, and tentacles.
- Mettaton may have artificial sexual parts.
- Napstablook may have genitals.
- Frisk and Chara may have both genitals(intersex).
- Frisk and Chara's age can also vary. Being a child, a teenager or an adult.
- Undyne's sex can be affected in both ways, but commonly being female.
- Flowey may or not have genitals.
- Temmie's appearance may change, having a teenage body or simply a normal body.
- Asriel's behavior may change depending of his forms. In his god-like form he can be more appealing and dominant, while in his child form he is more shy and passive.
- The Annoying Dog/Toby Fox is the least affected character in Undertail, while Sans is the most affected.
- The AUs most affected in the Undertail fandom are Underswap, Underlust and Underfell.