CA, his trouble is getting to his target through enemy infantries and allies models. Often you see him stuck with your own ally cavs around the target doing nothing. Stuck against chaffs or summons.
not get stunned in combat against monsters. Even in combat, he doesnt get a lot of attacks.
Green knight needs - immune to vigor. Being the first grail knight that could fight for week ends...
Swiftness of itzl to get out of chaffs and run faster towards target.
Decrease stun chance.
And even with these i doubt he will be very useful, so dont even worry about it being broken.
horse mount melee lords really suffer from this too
the best horse melee lord is tyrion which at least he has a chaff clearer to help himself and he is ONLY semi competitive
they are not good against foot lords/heroes since they get bounced around and couldnt surround it like cav units.
Bad against monsters cause they get stuck by allies/enemy units and get stunned.
Which makes their only good targets are other horse mount targets...
I honestly dont know how should that be fixed. Melee lords/heroes are generally meh compare to caster/hybrid/monsters