From the Super Mario Wiki
“Are the fell beasts gone? So it is safe for me to come out thither?”
When Mario first encountered Yebbi, he actually had not seen him in person. Yebbi was afraid of the monsters invading The Overthere and decided to hide in a restroom. When Mario knocked, Yebbi asked who he was. The responses available to the player were: "Mario!", "Grambi!", and, comically, "A monster!"
If the player chooses to respond, "Mario!", Yebbi declares, "Mario? What a strange name! Thou must be a monster!" If the player responds "Grambi!", Yebbi insists "Grambi? Thou liest! Thou must be a monster!" Then, ironically, when the player responds "A monster!", he shouts, "Thou LIEST! No monster would admit to being a monster! Which means... thou art not a monster?" Then, he comes out of the restroom.
Even when he first came out, he was unwilling to give the party the orb, but he reluctantly agreed when Luvbi threatened to tell everyone he had been hiding in the bathroom. He came out and relinquished the Yellow Orb, which was required to build a bridge to Grambi's castle.
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