They found each other in an alternate universe
You know they're soulmates if they can find each other in an alternate universe. That's why AU Fanfictions are the best kind.
They look at each other like two idiots in love
Basically every single time Simon and Izzy have a scene together the heart eyes come out strong. They may not realize they're in love, but their faces tell a different story.
Isabelle brought him back
When Simon was manipulated by Heidi into drinking his sister's blood, Isabelle was able to pull him back. It was her voice calling his name that brought him back to reality and saved his sister's life.
They're hilarious together
Everyone loves comedian soulmates. These two are already hilarious on their own, but together they're like comedy gold.
They already go on dates
For two people who are just friends, they sure do go on a lot of dates together. From dinner at the Hunter's Moon to moonlit strolls in the rain, they basically already act like a couple.
Isabelle knows Simon's heart
Despite what he was manipulated into doing to his family, Isabelle knows that's not who Simon is. She believed in him even when he didn't believe in himself.