So I was recently struggling with a similar problem when writing ahead in Errant.
After the prologue, we're about to do a timeskip, and after the timeskip, it will focus on more grown up versions of the characters and the cast that'd be familiar to readers of the old comic Errant is a reboot of, Fan Dan Go. In every version of FDG, Rekki, the main character, has always been partnered up with this guy called Subo. Subo is a chill, relaxed guy, he doesn't really like fighting and he's ended up drifting into a career as a knight because he's got magical abilities that make it easy for him. He's a gentle, chill guy with no particular ambition who everyone underestimates and he contrasts Rekki's hot-blooded intensity.
When I wrote the chapters though, I found that I just couldn't do anything with this legacy character. He was just there and had no real motivation or arc. I also found that the framing device I was using to reintroduce readers to an adult version of Rekki who is a celebrated knight but clearly really burned out using the framing that it's a news report on TV... wasn't working, because that kind of narrative framing felt jarring; too different from how Errant tends to be framed and too confusing.
So I tried writing a draft where Subo is the first character we meet, and I turned up his general chillness to eleven to make it a proper character flaw. So now we have this lazy guy who just wants to be a knight with minimal effort and hang out in the bar. He's not a bad person, but he clearly sees himself as unimportant and wants to leave important decisions to other people and just let the world drift by. Then BAM! Something happens that has him paired up with this struggling workaholic who is famous and works on the most high-profile, vital work, and he has to learn he's the only person who can do this and the people in charge are just humans making decisions that are often pretty poor.
So now Subo has an arc of his own and also serves two important roles: He's "The new guy" who I can introduce characters Rekki knows to, and he's "The normal guy" who reacts to all the ridiculous shounen crap in Rekki's life and also speaks and acts on behalf of the ordinary people in this big epic conflict.
So my advice would be to see if you need a character to explain things to or introduce characters to, and to also see if there's an element of his character you can exploit to give him an arc or an element that clashes with or contrasts other people. It could be that his hot-blooded, simplistic approach of "fight all the monsters!" puts him into a crisis when the conflict is more complex. It could be that he's always felt like he's owed the position of leader of a team of heroes having modelled himself on that character archetype and resents Selene having the spotlight. It could be he acts like a hero from a story/comic/game to cover up for some sort of shyness or to emulate a masculine ideal pushed on him by family or friends, but it's all an act. There are plenty of options.
Look at the pieces of your story, including characters, like a puzzle. Maybe you can't use the piece, but maybe you can use it if you turn it the right way.