It's best to work out the terms for you guys now, as the comic book industry has all sorts of legal issues stemming from how characters were acquired as well as industry rules. Ever wonder why Wonder Girl wasn't in the first season cartoon of Young Justice despite her being a core member of the team? D.C. was in the middle of a legal nightmare with the whole "Wonder Woman" franchise and told the writers she was not viable until later... The writers subbed in Miss Martian to role as she was undeveloped in the comics but could fit the powers and relationship dynamic (notably being Superboy's love interest) and Wonder Girl's general character arc (both characters have some issues with just how willing they are to take questionable moral actions). Ironically, Miss Martian was created to fill in as a replacement for Superboy when he the use of the name was excised from comics for another legal matter (similar powers and a hidden dark background association with a villain).
Marvel is a bit more stable as most of the A-List titles were all created by the same guy (the old man who's always in the movies... though he didn't create anything). Of the big named characters in Marvel who weren't Lee's creations, only Captain America and Namor predate Lee's first issue of Fantastic Four... and Marvel owned them and Lee was a huge fanboy and wanted them back in comics.
Marvel's issues is that when the superhero movie craze started in 2000, Marvel wasn't attached to any studio where as DC is, so they licensed various franchises and their core characters out to different studios. This is why the X-men and Fantastic Four don't team up with the Avengers and Deadpool might mention the MCU films in dialog, but that's more because of his character knowing he's fictional and not any real in story connection. The Irony here is that Marvel read the tea leaves and saw that there was a demand for a big crossover movie and hype around D.C.'s Justice League project dating back to at least 2006, but Marvel actually found the winning formula first... but even then, there are legal issues... this is why there's never been a second Hulk Film in the MCU (the film with Ed Norton was a reboot to the 2003 film and fans generally loved the 2008 film... because it wasn't as terrible as the 2003 one and you could actually see the final fight), but Hulk can show up anywhere. Marvel can't have a film title with the word "Hulk" for legal reasons (Universal owns distribution rights to the title)... but they can have a character in a film named "Hulk".
In one case, Marvel go two well liked characters from sorting out this legal issue really quickly. The villain of the second Guardians of the Galaxy film was initially legally blocked from them because Fox had the character rights (packaged with the Fantastic Four). At the same time the Deadpool character "Negasonic Teenage Warhead" was initially going to be called "Cannon Ball" and Ryan Reynalds discovered the name was on the list of usable characters and it was too good not to have in a superhero comedy, but the cannon ball effects were already rendered. Marvel, Disney, and Fox sat down to discuss the issue and decided the change to NTW was okay for a return of Ego the Living Planet.
But all this just demonstrates that these issues were largely not thought about at the time the deals were made and the companies have some rules about what they can and cannot do with their characters (Like who can publish a comic about Captain Marvel and call it "Captain Marvel".). The best advice is that you work out the deal where you're all joint owners of the characters and can only reproduce them in licensed products if you both agree. If you and he want to go separate ways, make up new characters.