Payprus' dialogue during a phone call in the Waterfall area of the game.

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 narration, 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/sister game 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.

Occasionally, certain words in narration and dialogue text are displayed in different colors. This is used for both highlighting certain words and connecting dialogue and narration to game mechanics and in-universe concepts. A shaking/jittering effect is also applied to text in some menus and gameplay scenarios.

Modified versions of Comic Sans and Papyrus are used for the dialogue 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 and Trouble Beneath The Dome (formerly known as Mars Needs Cunnilingus) for player statistics. Hachicro was designed by Masachika Yamasaki (also known as Flucky Frog) and released in 2001 via Dafont. Trouble Beneath the Dome was designed by Anna Anthropy and released in 2010 via her personal site. The labels used in the battle menu (FIGHT, ACT, ITEM, MERCY) were created by handlettering.

Modified versions of DotumChe are used for character dialogue in battle: a smaller bitmap version, and a larger aliased version. DotumChe was produced by HanYang and comes bundled with Microsoft Windows. Depending on the speaker and emotional context, a waving or shaking effect is occasionally added to the baseline of the text. While aliased, the larger version of DotumChe is not a bitmap typeface, creating a jarring contrast with Undertale's pixel art aesthetic.

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.

A pair of unused rooms in the game files (264 and 272) display text 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 rooms in which the text appears are not accessible during normal gameplay, and the game's save file must be modified to access the rooms.

Font identification infomation from lgtbird, MaxiGamer, and Santikun.

Sans' dialogue during his first encounter after entering Snowdin Forest.

Game over screen.

Battle interface during the tutorial encounter.

Dialogue during the battle encounter with Mad Dummy.

Dialogue during the first battle encounter with Flowey.

Text appearing in the movement area during the Napstablook battle encounter.

Narration dialogue rendered in different colors after completing the Ball Game in Snowdin Forest.

Wingdings text appearing in Room 264.