Glyde is an optional miniboss that can be encountered outside of the Mysterious Door in Snowdin Forest. It was designed by Mike Reid as a bought-in character during Undertale's Kickstarter.

To encounter Glyde, the protagonist must enter the Mysterious Door's cave and move for at least two minutes. This encounter must happen before the fight with Papyrus, and if any fleealbe Snowdin enemies were not killed. Otherwise, Glyde does not appear. Alternatively, if at least one monster was killed, the encounter must happen before the date with Papyrus and before Snowdin Forest's kill counter is exhausted at 16 kills.

Glyde is excessively pretentious and rude, which is present through its actions and words. Glyde does anything to get the protagonist's attention, and even gets jealous of itself at times.