He speaks improper English, pronouncing "s" as "th". When the Beanstar broke into four pieces, Hermie III found one and placed it at the top of his shell, which made three of his fans come to help him decorate it more. When Mario and Luigi came, the fans paid more attention to them, making Hermie III furious.
Hermie III can also be seen during the ending sequence where he is waving to the plane as Mario and Luigi head back to the Mushroom Kingdom.
Hermie III has three attacks. His basic attack is trying to pinch the Mario Bros. with his claws, which can be avoided by jumping. Damaging the claws prevents him from using his claw attack. He also spits out purple bubbles at Mario and Luigi. These bubbles must be countered with hammers or else the Bros. become poisoned. After the Mario Bros. hit Hermie III for a while, the boss will duck into his shell and heal during his turns. To get him out of his shell, attack with the Firebrand. In the 3DS remake, he has an additional gimmick: if you avoid his pinching attack perfectly, Hermie III increases the speed at which the attack will move.
After a long battle with Hermie III, he gives Mario and Luigi his piece of the Beanstar after Mario and Luigi tell him all about the situation. When Mario and Luigi bring him Spangle as a substitute to the Beanstar, Hermie III gives them the Soulful Bros. badge.
Hermie III is the only boss in Mario and Luigi: Superstar Saga who can be easily defeated just with the use of Mario's Firebrand (which burns his shell) and Luigi's Thunderhand (which deals critical damage to him).
Due to the Christmas decorations on his shell and the appearance of Mario (wearing red and being a little chubby), Hermie III thought that Mario was Santa Claus. ("C-could you be... SANTA CLAUS!?")
Hermie III earned his name from Hermit Crabs, which he slightly resembles. The III is a reference to the Roman numeral III, and thus a pun on trees.