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Good Star is a deathless power star, created by SMG3 and Bowser to match the skills of the Ztar, in the blooper Bad Stars. Instead of being half Toad , this star was half Old Man. It never actually defeated the bad star, but after Mario knocked it out using a Toad, the Ztar got tied up on a boat, with the good star 'guarding' it forever.
It soon make a re-appearance in "Bad Stars Back" where Ztar frees itself and attacks Good Star. Good Star later is mentioned to take a bath and will take at least 20 minutes to take on Ztar again. Despite this, it quickly comes to the rescue in at least a minute disguising as a normal star. It's last seen defeating Ztar by sucking it into a portal with itself.