that's a nice addition but not what I tried to say (and obviously failed).
in the pre-battle screen available chars (the vertical bar on the left with 6 slots) shows mandatory chars with a yellow name. it is easy to see which chars have to be active. the marker is missing in the active chars bar (the horizontal bar lower middle with 4 slots).
I was under the impression, that the main char has to be active in all encounters, with the exception of arena fights. but this is not true, something I only realized because of a screen shot you posted with a boss fight and no MC active.
I even have a relevant use case for this: it would be nice to see in multi-part encounters which chars can be swapped out to heal. at the moment one has to deactivate them to learn if they are mandatory or not.
(last try to explain this, I'll shut up now. promise!)