Shifting focus to Selena seems like a very viable solution.
As for Flare, I sometimes think of fiction characters as if they were RPG characters. Once in a while, a player will say to me something like, "Yeah, I have this character, but she's not very interesting. She's just a standard (elven thief). I want to give her a backstory before I roleplay her."
My response to this is: "Our first adventure is her backstory. Just play the game. Let her get shaped by her experiences at the table, and she will become an interesting character for you."
So yeah, just write some stories that include Flare. These experiences can expand his personality and complexity just as his power increases with every new kaiju that he has to face down.