If you start Crimson Scales scenario 25: Brightspark Behold with a mercenary Brightspark in your party your mercenary becomes a second copy of the scenario NPC Brightspark. This is also permanent so even switching to a new scenario your mercenary will still be a NPC Brightspark :D.
I had to remove him from party and readd a new Brightspark to fix the issue.
If you start Crimson Scales scenario 25: Brightspark Behold with a mercenary Brightspark in your party your mercenary becomes a second copy of the scenario NPC Brightspark. This is also permanent so even switching to a new scenario your mercenary will still be a NPC Brightspark :D. I had to remove him from party and readd a new Brightspark to fix the issue.