Huh? There's a letter lying right in the middle of the road!

The letter was written as follows... 💌 If you have picked up this letter, it means the world's boin has begun to vanish... Guest... you are the only one who can stop the rampage of Arch-Chancellor Pettana of the evil flat organization [Flat-Chest Syndicate]... Take the Rainbow Boin Diamond inside the envelope and go to the candy store. That Rainbow Boin Diamond is a one-of-a-kind rare diamond that is guaranteed to produce one random Boin character. (Please look around various places for regular Boin Diamonds❤) Use that Rainbow Boin Diamond to spin the gacha❤ Now, hurry! The sender's name is nowhere to be found on the letter...