Definitely Mario and Sans. Sans' laid - back attitude and his powerful abilities would contrast greatly with Mario's more heroic and straightforward nature. Their conversations would be filled with humor and misunderstandings. For example, Sans might make some jokes about Mario's mustache or his choice of overalls, while Mario would be confused by Sans' skeletal appearance and his knowledge of timelines.
I think the interaction between Luigi and Napstablook would be really interesting. Luigi is often the more timid of the Mario brothers, and Napstablook is a very melancholic character. Luigi might try to cheer up Napstablook, and in the process, they could form an unlikely friendship. Maybe Luigi could introduce Napstablook to some of the fun things in the Mushroom Kingdom, like power - ups or the Toads' parties.
Definitely Sans and Jevil. Sans' laid - back yet perceptive nature combined with Jevil's chaotic and unpredictable personality would make for some really interesting exchanges. Sans might try to understand Jevil's view of the world, which is so different from his own in Undertale.
Definitely Joker and Makoto Yuki. Joker is the leader of the Phantom Thieves, very confident and strategic, while Makoto Yuki is a strong and silent type. Their leadership styles could clash or complement each other.
Definitely the interactions between the two versions of Leonardo. The 2012 Leo is more light - hearted in some ways compared to the very serious 2003 Leo. They could have some great moments of leadership clashes and then learning from each other. For example, the 2003 Leo could teach the 2012 Leo some of his more stoic leadership techniques.
I think the interaction between Joker (from Persona 5) and Yu (from Persona 4) would be really interesting. They are both leaders of their respective groups, so they could exchange leadership strategies and learn from each other. Also, Morgana and Teddie would be hilarious together. Morgana's serious yet sometimes clueless nature and Teddie's over - the - top and comical personality would create some great comedic moments.
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.
Alphys and Iron Man. Alphys is a scientist in Undertale who is a bit of a klutz but also brilliant. Iron Man is a genius inventor in Marvel. They could collaborate on some crazy technological inventions. Alphys could learn a lot from Iron Man about presenting herself more confidently, and Iron Man could be intrigued by Alphys' unique approach to science that comes from the Undertale world.
Frisk could be a very interesting character to focus on. Frisk has the power of determination, which could be used in unique ways in the FNAF setting. They could befriend some of the animatronics, using their kindness and determination. Also, since Frisk is the player - controlled character in Undertale, it would be interesting to see how they adapt to the more dangerous and unpredictable world of FNAF.
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.
I think Monika would be really interesting. In DDLC, she has that self - awareness and a certain level of power over her world. If she were in Undertale, her attempts to understand the very different rules of that world would be fascinating. She might try to use her powers to change things, but in Undertale, the concept of determination and the power of the player is very different. So seeing her interact with characters like Flowey, who also has some unique powers and knowledge, could lead to some really intense and thought - provoking scenes.
Sans is another great choice. His laid - back but extremely powerful nature in Undertale would create a lot of interesting dynamics in a Deltarune world. He could befriend or baffle the Deltarune characters. For example, his time - related abilities would be very different from anything in Deltarune, and it would be fascinating to see how he uses them in a crossover situation, like against the Darkners.