From the Super Mario Wiki
Chester is a gooey monster disguised as a purple Treasure Chest that appears in Super Mario RPG: Legend of the Seven Stars. Inside the chest is its body, which resembles a tan Ameboid. It is one of the various monsters spawned by Magikoopa and can be found in behind one of the six doors in Bowser's Keep.
Chester is the most powerful member of the Pandorite family, sporting the highest stats in all areas, but also follows a predictable pattern. Its first action in battle will be to summon a Bahamutt. On its next turn, Chester will use Sandstorm to damage the entire party, as well as possibly inflict Fear. On its following turn, it will use either Mega Recover to heal or Flame Wall to damage the party. The odds of each is 2/3 in favor of Mega Recover, and 1/3 in favor of Flame Wall. Chester's pattern then starts over, with the difference of checking to see if Bahamutt is still active: if not, it will summon another one, but if it is, Chester will use Flame Stone to inflict Fire damage to one character.
Like its brethren, Chester is immune to all elemental attacks and status effects except Jump, which it is weak to. It is best to have Mario focus on Chester, using Ultra Jump to inflict as much damage as possible each turn. Non-elemental group attacks such as Bowser's Bowser Crush, Geno's Geno Flash or Geno Blast, or Princess Toadstool's Psych Bomb can be used to damage both monsters simultaneously.
A Chester can be found long before the one in Bowser's Keep, inside Monstermama's house in Monstro Town; however, it is not an enemy. It appears in the upper room and tells Mario how many invisible chests (or "Surprise Boxes" as it refers to them) he hasn't found in the game; however, collecting them all only prompts him to say, "Wow, you found them all."
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