I think Sakura and Toriel could also be a popular pairing. Sakura's strong - willed and healer - like qualities would mesh well with Toriel's motherly and protective nature. They could potentially team up to take care of the injured or those in need in a combined Naruto - Undertale world.
Naruto and Sans are quite a popular pairing. Naruto's energetic and determined nature contrasts with Sans' laid - back and somewhat mysterious attitude, making for an interesting dynamic. They could bond over their shared experiences of protecting their respective worlds.
Another popular pairing is Sasuke and Papyrus. Sasuke's brooding and powerful presence paired with Papyrus' goofy yet kind - hearted personality creates a really unique relationship. They could learn a lot from each other, Sasuke about kindness and Papyrus about power and self - improvement.
Sans from Undertale and his Underswap counterpart are often a popular pairing. People like to explore how the two different versions of him interact, given their different personalities. Another popular one is Papyrus from Undertale and his Underswap version. They can have some really funny exchanges.
Naruto and Frisk would be really interesting. Naruto is all about never giving up and has a strong sense of justice, while Frisk is determined in their own way, always choosing the path of peace and kindness in Undertale. Their combined determination could lead to some great adventures and they could learn a lot from each other's ways of dealing with challenges.
Ruby and Sans are quite popular. Ruby's energetic and positive personality contrasts well with Sans' lazy - yet - deep - thinking nature. Their interactions can be really interesting as they come from such different worlds but share a certain sense of humor. Another popular pairing is Weiss and Papyrus. Weiss' elegance and determination can play off Papyrus' over - the - top confidence and his own sense of honor in a really engaging way.
One popular pairing is Naruto and Link. They are both heroes in their respective worlds, and a fanfiction might explore how they would interact, train together, and learn from each other's fighting styles.
Naruto and Ichigo are often a popular pairing. They are the main protagonists of their respective series and fans like to imagine how they would interact. Their different yet powerful abilities make for an exciting combination.
Some fanfictions also explore the pairing of Naruto (female) with Neji. Given Neji's initial views on the Hyuga clan's hierarchy and Naruto's status as an outsider (even more so as a female in a male - dominated ninja world), their relationship could be full of conflict and resolution. It could be about breaking down barriers between them both in terms of clan differences and gender roles.
Another popular pairing is Naruto and Hinata. Given their relationship in the original Naruto story, fanfiction often delves deeper into their married life, how they raise their family, and the challenges they face as a couple in the ninja world.
Ruby and Frisk would be a really interesting pairing. Ruby is full of energy and determination, and Frisk has that unwavering kindness. They could learn a lot from each other, especially in terms of how they approach difficult situations and make decisions.
Susie and Undyne would be a great pair. Susie is the tough - looking monster from Deltarune, and Undyne is the powerful fish - like monster in Undertale. They could have an epic battle at first, but then realize they have a lot in common. They both have a strong sense of justice and a love for fighting. After their initial clash, they could team up to fight a greater evil, perhaps something that threatens both Deltarune and Undertale. They could exchange fighting techniques and learn from each other's strengths.
I think Leonardo and Papyrus would make an interesting pairing. Leonardo's leadership and discipline could contrast with Papyrus' more naive and enthusiastic nature. They could learn from each other, like Leonardo teaching Papyrus some ninja moves, and Papyrus showing Leonardo how to have more fun. Another great pairing could be Donatello and Alphys. They are both the 'tech - smart' characters in their respective universes. They could collaborate on some crazy inventions that combine Undertale technology and Turtle - made gadgets.