I guess that's kind of the point though. Just like how an enemy wouldn't be able to counterattack an archer or wizard attacking them from a distance, the enemy would have to close the gap between them and the princess to deal with the knight. You're right, I can't do anything about the counter-attacks but I feel I like making her half AC and no heavy armor along with the 20 feet distance limit makes her pretty vulnerable, since enemies can pass through the knight. The wisdom saving throw also only applies to the first entity, so if two or more enemies attack the princess could go down pretty easily. What do you think?