From Wikipedia:
Undertale is a 2015 2D role-playing video game created by American indie developer Toby Fox. The player controls a child who has fallen into the Underground: a large, secluded region under the surface of the Earth, separated by a magical barrier. The player meets various monsters during the journey back to the surface, although some monsters might engage the player in a fight. The combat system involves the player navigating through mini-bullet hell attacks by the opponent. They can opt to pacify or subdue monsters in order to spare them instead of killing them. These choices affect the game, with the dialogue, characters, and story changing based on outcomes.
Undertale uses the typeface 8-Bit Operator JVE for overworld dialogue and as the primary interface typeface. 8-Bit Operator JVE was designed by Jayvee Enaguas (also known as harvettfox96 or nipcen, among other online names) and released in 2011 via Dafont. Tighter kerning is used in menus, and looser kerning is used for dialogue and narration. The 8-Bit Operator family is also used for branding and identity for Undertale and its successor Deltarune.
Due to letterform similarities, 8-Bit Operator JVE is often mistaken for Fixedsys, other variations of 8-Bit Operator, and Pixel Operator, the successor to the 8-Bit Operator font family. In 2015, hobbyist Haley Wakamatsu (also known as JapanYoshi) released the font family Determination, a clone of 8-Bit Operator JVE based on an Undertale spritesheet compiled by Leafia_Barrett.
Modified versions of Comic Sans and Papyrus are used for the dialog of the characters Sans and Papyrus respectively. Comic Sans was designed by Vincent Connare and released in 1995 by Microsoft. Papyrus was designed by Chris Costello and released in 1983 by Letraset. Both typefaces have been modified into bitmap type to fit Undertale’s pixel art aesthetic.
The overworld interface uses the typeface Crypt of Tomorrow for displaying player statistics. Crypt of Tomorrow was designed by Anna Anthropy (also known as Auntie Pixelante) and released in 2008 via her personal site.
The battle interface uses Hachicro for damage amount numbers, Mars Needs Cunnilingus for player statistics, and a modified version of DotumChe for character dialogue. Hachicro was designed by Masachika Yamasaki (also known as Flucky Frog) and released in 2001 via Dafont. Mars Needs Cunnilingus was designed by Anna Anthropy and released in 2010 via her personal site. DotumChe was produced by HanYang and comes bundled with Microsoft Windows.
8-Bit Wonder is used as the main typeface on the Game Over screen and for the “HP” text on the battle interface. 8-Bit Wonder was designed by Joiro Hatgaya and released in 2001 via Typesource. After Typesource became defunct, Hatgaya re-released 8-Bit Wonder in 2009 via Dafont.
During the Napstablook battle, the typeface Slanted is used for text that appears in the movement area. Slanted was designed by Anna Anthropy and released in 2009 via her personal site.
An unused room in the game files displays dialogue in a modified version of Wingdings. Wingdings was designed by Charles Bigelow and Kris Holmes and released in 1992 by Microsoft. Similarly to Comic Sans and Papyrus, Wingdings is modified into bitmap type. The room in which the text appears is not accessible during normal gameplay, and the game's save file must be modified to encounter it.
Font identification infomation from lgtbird, MaxiGamer, and Santikun.