Fuegoleon was really surprised to learn Mereoleona was the captain of the Crimson Lions and pointed out that she isn't really into the whole magic knight thing in general. The narrative also seems to heavily imply that the Crimson Lions captaincy might be an inherited captaincy. Fuegoleon took over from their dad, Leopold seems super certain he will be captain one day, etc... So I'm thinking Mereoleona took over even though it's not really her thing not only because revenge, but possibly also because otherwise Leopold would become captain and he’s only 16 or 17 depending on when August is in the storyline so he's been at this magic knight thing for months or over a year most likely. So like green teen child soldier leading an elite squad is a bit much (and we see this in how Leopold loses control of the situation when he doesn't retreat with the injured). Why put her littlest brother through a captaincy during an invasion from Eye of the Midnight Sun when he’s ill-equipped for it emotionally? That’s like Big Sister 101 especially after Fuegoleon ended up unconscious and amputated.