>>538899921
This screenshot has both blue and green units "line" units grayed out.

Maybe they act first this time, or you can actually control green units here (thus the blue portraits) but they're highlighted like that to show they won't stay in your party? Note the green units here in this Cai shot seem to be from Dietrich, another protagonist, and it's likely each one would have their own route, being unable to recruit other protagonists.