Saroh and Zupyrus

Sans as Uncle Iroh and Papyrus as Zuko
...
seriously, think about it

Sans & Iroh:
- small and chubby (looking)
- funny, at least to themselves and their fans
- more powerful than one would at first think
- knows more than you'd expect at first
- knows even about things that go above the perception of many (Sans: timelines & resets; Iroh: Spirit world)
- love a certain kind of drink (I think you can count Sans' ketchup as a drink after the amount of fanart picturing him drinking from a ketchup bottle)

Papyrus & Zuko:
- train eagerly for their goal
- want to gain respect, recognition, love, honor
- want to do this by capturing someone (Papyrus: human; Zuko: Avatar)
- also want to be able to 'join' somewhere by doing that (Papyrus: Royal Guards; Zuko: Fire Nation, specifically his family)
- are sometimes blind for the obvious due to their own eagerness/obsession

Undertale belongs to Toby Fox
Avatar - The Last Airbender belongs to Nickelodeon