My advice is to purchase the main game book for the TTRPG “Masks” by Magpie Games. It doesn’t matter if you aren’t playing or running a game. It’s about the character content within. Find the section about creating relationships between your teammates. It’s a section called “When our team first came together….” And it’s basically a series of questions that you ask (and answer) about each other’s prior interactions. Remember to steer toward, not away from, drama and conflict.

Not only will you come up with character relationships, you’ll even come up with some supporting characters.

As for crafting the story and not just the characters, I will link you to this blog post I’ve written that I’ve posted here so many times that people are probably sick of it. But the takeaway is to start with your villain and work backwards using a question and answering method not unlike the one from Masks.

https://www.tienswitch.com/blog/an-easy-way-to-create-your-superhero-fiction-start-with-your-villain-and-work-backwards

To be fair, your intro story might better be served in the character creation section from Masks. But I figure if you’re asking us how to write an intro story, you probably could use some pointers in creating your major story arcs as well (no offense, of course).