Undertale

Undertale is a roleplaying video game developed and published by Toby Fox.

Plot
Long ago, monsters and humans ruled the Earth together in peace. One day, war broke out, and after a hard and long-fought battle, the monsters were banished to the underground, a massive cave large enough to have its own weather system. They were sealed in by a magic barrier that was at the top of Mt. Ebott, created by the human's seven greatest magicians. Anyone can enter through the barrier, but only something with an incredibly powerful soul could make it out. You play as the human Frisk, who has fallen through the barrier, and strives to make their way out. The are many different monsters, who can be friendly or evil. You can make many friends, or you can kill them all. But be warned-if you kill to much, you may lose something that is impossible to get back…

More about the plot at Underground - Undertale Wikia

Fandom
Undertale is a very popular game due to its unique design and long, creative story. Some people do not like it because it is "old school," but many fans believe that it is very unique, and almost nothing like an "old school" style rpg.

Canon

 * Alphyne - Alphys and Undyne
 * Asgoriel - Asgore and Toriel

Non-Canon (Fanmade)

 * Chans - Chara and Sans
 * Frans - Frisk and Sans
 * Papyton - Papyrus and Mettaton
 * Undyrus - Undyne and Papyrus
 * Frisriel - Frisk and Asriel
 * Chasriel - Chara and Asriel
 * Soriel - Sans and Toriel
 * Charisk - Chara and Frisk
 * Papyrisk - Papyrus and Frisk
 * Sansby - Sans and Grillby
 * Flowisk - Flowey and Frisk
 * Blookyton - Nabstablook and Mettaton
 * Flowtem - Temmie and Flowey
 * Sanphys - Sans and Alphys
 * Gastara - Gaster and Chara
 * Frisemmie - Frisk and Temmie
 * Frisk Kid - Frisk and Monster kid
 * Friskablook - Frisk and Napstablook