I noticed a funny behavior from battle sprites: they can't be used when you're charging an attack.
Being a player who mostly uses charged attacks (read: charging Blitz Needle / Shiver continuously when fighting Vanaduke), I find my mer... shadow sprite quite useless. At least at its early levels. I took it for the poison status effect, hoping to have a free poison to vana right before shooting my bunch of arrows, but it is out of question - there's just no time for me to pause and use sprite in between my charged attacks (bomb - gun - bomb - gun).
Making knight able to use Battle Sprite companion whenever they desire so, and not only when they're NOT in a fight, would be of great benefit to every sprite user (in other words - all knights).
For a moment I thought about putting it into "bugs" section...
Yes, this is a good idea.