I think Ruby and a young, adventurous Pilot would make a great pairing. Ruby's enthusiasm and eagerness to learn new things would mesh well with a Pilot's daredevil nature. They could exchange stories about their respective worlds while fighting side by side.
Zero and Ruby would be a likely pairing. Zero is a powerful and skilled fighter, and Ruby has a lot of potential and a unique weapon. They could learn from each other's combat styles. Ruby could learn about Zero's more serious and battle - hardened approach, while Zero could be inspired by Ruby's enthusiasm and creativity in fighting.
Kirby and Yang could also make an interesting pairing. Yang is strong and brash, while Kirby is small but mighty. Their personalities contrast but could also complement each other well. Yang could provide the brute force while Kirby could use his unique powers to support her in battles, like when she's facing a particularly tough Grimm.
Ruby could also be a great candidate. Her natural leadership skills and her ability to quickly adapt to new situations would serve her well. Plus, her speed and agility could make her an excellent Pilot. She might even come up with new strategies for using the Titans in combat, inspired by her own experiences in RWBY.
I think Harry and Blake could also be paired well. Blake's intelligence and her past experiences with the White Fang could be complemented by Harry's knowledge of fighting dark forces. They could bond over their shared experiences of fighting against those who want to bring harm and chaos, whether it's the Grimm in Remnant or Voldemort in the wizarding world.
Sans and Ruby are quite popular. Ruby's enthusiasm and Sans' nonchalant attitude can create a really interesting dynamic. They're both important characters in their respective worlds, so fans like to explore how they would interact.
Sasuke and Erza could be a likely pairing. Erza is a strong and disciplined character in Fairy Tail, much like Sasuke in his own way. They could bond over their combat skills and their determination to protect what they hold dear. Another possible pairing is Sasuke and Lucy. Lucy's kind - hearted nature might contrast with Sasuke's brooding personality, and their relationship could be one of growth and understanding for both characters.
The Doom Slayer and Ruby would be an interesting pairing. Ruby is young and full of energy and hope, while the Doom Slayer is a silent, stoic force of destruction against evil. Their interactions could be really cool, like Ruby trying to understand the Doom Slayer's lack of words but seeing his determination and being inspired by it. And the Doom Slayer might see Ruby's potential and unwavering spirit in the face of danger as something similar to his own.
Ruby and Noel would be a great pairing. Ruby is young, energetic, and has a strong sense of justice. Noel is also a very determined and kind - hearted character. They could bond over their shared values and also learn from each other's combat styles. Ruby could show Noel some of her acrobatic moves with Crescent Rose, while Noel could teach Ruby some of the finer points of her swordsmanship.
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.