Poor Axel. Like Roxas, he just wants a purpose for living, even if that purpose happens to be eating bright blue ice cream on a rooftop with friends. Kingdom Hearts 3D: Dream Drop Distance finally gives him a purpose, but not what you'd expect.
Despite working for some form of good throughout the series, everything ends up going badly for Axel. He’s torn between protecting one friend or another, loses them both, potentially develops some feelings for Kairi—possibly because of her connection with Xion — but naturally can’t act on them, all before having to give up himself and his memories so Lea, the "real" Axel, can be revived.
His extensive history in the series gives the player a lot more investment in Lea than would otherwise have been the case, so the lead-up to this moment is much more engaging than it would be for your average NPC. Lea ends up responsible for saving Sora yet again, for rescuing Queen Minnie, and basically for keeping the entire world from falling apart.
That he finally gains a Keyblade for his trouble is a fitting reward for his actions and all Axel endured, but it also means he can fight to right the wrongs done to him and all of his friends, Nobody and normal, finally starting to tie up the many, many loose ends KH dropped all over the place.