It could be Sonic and Lucy. Lucy is often the first to encounter new things or people in the Fairy Tail world. She would be curious about Sonic, his strange appearance and his abilities. Maybe Sonic is running through town and bumps into Lucy, and that's how their interaction starts. She could then introduce him to the rest of the guild.
Sonic and Natsu would be likely to interact first. Natsu is always eager for a challenge and when he sees Sonic's speed, he would be intrigued and want to test himself against Sonic.
Hulk might be an interesting one too. Sonic's speed could either annoy or amuse Hulk. If they end up on the same side, Sonic could use his speed to lead Hulk towards the enemies. And Hulk's raw strength could be something Sonic would be in awe of. They could have a really unique dynamic, like a fast - moving pest annoying a big, strong giant at first, but then they become an effective team.
Gray is another candidate. Gray's cool and collected demeanor could contrast with Tobi's more mysterious nature. They could get into arguments about the best way to handle a situation, like when facing a powerful enemy. Or, they could find common ground in their skills and work together to protect Fairy Tail from danger.
Happy from Fairy Tail and Pinkie Pie from My Little Pony. Happy is a fun - loving and friendly cat - like creature, and Pinkie Pie is the life of the party. They would likely find each other immediately and start having a great time together, sharing jokes and treats. Pinkie Pie might even teach Happy some of her party - planning skills.
Applejack and the Heavy might be the first to interact. Applejack is a hard - working and practical character. The Heavy is a no - nonsense fighter. Applejack might see the Heavy and be curious about his large size and strange weapon. The Heavy, on the other hand, might be interested in Applejack's farming skills and the fresh apples she might be carrying. Their down - to - earth personalities could lead to an initial interaction.
One popular pairing could be Sonic and Lucy. Their personalities are quite different, with Sonic being adventurous and fast - paced, and Lucy being more of a kind - hearted and sometimes shy mage. The contrast between Sonic's speed and Lucy's celestial spirit magic can make for an interesting dynamic in fanfiction stories. Another might be Knuckles and Erza. Knuckles' strength and loyalty paired with Erza's leadership and combat skills could create some really cool storylines.
Probably Naruto and Robin. Robin is the leader - type in Young Justice, always thinking and strategizing. When Naruto shows up, Robin would be the one to approach him first, trying to figure out who he is and what his intentions are. Naruto, being friendly and curious, would be open to the interaction.
Natsu would be a likely candidate. He's always up for a challenge and not afraid to take on powerful opponents. He'd probably be the first to rush towards Godzilla, either to fight it or try to befriend it in his own Natsu - like way.
Probably Naruto and Tracer. Naruto is always eager to meet new people and make friends, and Tracer's curious and friendly nature would make her approach Naruto. They could start by comparing their abilities. Naruto would show off his Rasengan while Tracer might do a quick display of her blinking ability.
Probably Ash and Iron Man. Iron Man is always curious about new things and Ash with his Pokémon would be quite an interesting sight for him. He'd likely approach Ash first to ask questions about what he is and what those strange creatures are.
It could be Percy and Black Widow. Natasha is a master of reading people, and she'd notice Percy right away. She might approach him in a more stealthy way compared to the others, trying to assess if he's a threat or an ally. And Percy, being used to danger, would be on his guard at first, but then start to trust her as she shows her good intentions.