Used for the (overworld?) dialogue of none other than the great and shattered Dr. WD Gaster. Note: the Undertale version of Wingdings has different symbos than usual for specifically the apostrophe & quotation mark characters.
Used in Undertale for Papyrus's text bubble (only) within fights.
Used for the overworld dialogue boxes of (who else?) Papyrus.
Used for Sans's dialogue, but only in-battle. Note: the spacing on the upper-case characters is pretty much pure guesswork.
Used in the overworld dialogue boxes for Sans (pretty obviously).
Used in Undertale for the damage display (i.e. to show how much damage you dealt with an attack).
Used in Undertale for the player stats in-battle (only).
This font is used only once in Undertale. Whenever Flowey says DIE. Thus, I really didn't have a lot to go off of to figure out the character spacing for this one...
The font used in almost all the (in-battle specifically) Text Bubbles in Undertale.
Used in Undertale for the player stats in the menu.
Used in Undertale for most parts of the menus.
The main font used in Undertale. Used for almost all overworld dialogue.