Welcome to Hetatale!
First off, as you know, Hetalia and Undertale don't belong to me. Any crossovers of characters/any character, really, belongs to their respectful owners. Since Hetalia and Undertale are quite known, you probably know who made them, right? If not, Hetalia is made by Hidekaz Himaruya, and Undertale was made by Toby Fox.
First off, this is a purely made crossover made for fun. To twist it a bit, I will somewhat make headcanons for Hetatale, but some headcanons and canon things will fall in line. Since this part will be talking about the True Pacifist Route, I will not include any Genocide Route characters, including Chara and whatnot. Please keep in mind that Admin doesn't know a lot of stuff from Undertale, and Admin will probably never fix any mistakes given in the next few chapters. So please, no harsh criticism? :'D
Anyways, the characters presented are (Undertale = Hetalia):
Frisk will be Sealand
Toriel will be Hungary
Asgore will be Nyo!Ukraine
Flowey/Asriel will be 2p!Canada/Canada
Sans will be America
Papyrus will be England
Undyne will be Nyo!Germany
Alphys will be Nyo!Italy
Mettaton/Mettaton EX will be Prussia
NOTE THAT: This story will only use main characters. Minor characters will be portrayed as how I see them (Dogs = 2p!Greece, only nicer). The story will also use the countries human names.