you need the henshin jo for that
you get the henshin jo by having raised 4 characters who have all collected all 4 of the tamatomo stamps
every character can get 4 tamatomo stamps, 3 of the tamatomo stamps are collected by using an item/food/clothes/an accessory, and the last tamatomo stamp is collected by keeping the adult character for 48 hours.
you need to raise 4 characters, and they all need to have collected all the 4 tamatomo stamps. when you are at the henshin jo, you use an item to get a special character.
some extra info:
-a generation is when a tama is alive on your id l (sort of), everytime a new tama gets born by marrying your adult character, a new generation starts.
-the four characters you need to raise (with all 4 tamatomo stamps) don't have to be raised in a row. what i mean by this is that you don't need to raise the characters for 4 generations in a row. here's an example: you raise 3 characters with all their tamatomo stamps for 3 generations in a row, then you raise a character, but let it marry without letting it get all 4 tamatomo stamps. if that happens, it's ok!
you don't need to raise the 4 characters with all their 4 tamatomo stamps in a row, from the moment you have raised 4 characters with all their 4 tamatomo stamps, henshin jo becomes available.
-every tama gets 3 of their 4 tamatomo stamps by an item/food/clothes/accessory, you can find those items on the internet. each character has their own stuff that gives them their tamatomo stamps. so you can't use the same items on all the characters to get all their 4 tamatomo stamps, every character has different items to get their tamatomo stamps, you can find which character needs which items on the internet.
-the last tamatomo stamp is collected by having your adult tamagothci for 48 hours. when your teenager character evolves, you get an adult character. after you keep this adult character for 48 hours, it will get its last tamatomo stamp.
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not all characters can change into special characters
-when you have gotten the henshin jo, you need a specific item to get a special character. every character which can evolve into a special character, needs a different item to change into the special character. you can get some of these items on the tamagotchi ID L, and some of the items have to be downloaded from a japanese site, if i remember corrcetly, you need a phone with IR connection, or an irda usb stick.
- when you have the henshin jo, and a character that can change into a special character, and you have the item it needs to change into the special character, you need to go to the henshin jo, which you can go to by going to the door icon, and then choosing "henshin jo". there, you need to use the item on your character, and it will change into the special character.
- every 4 generations of characters that have gotten all their tamatomo stamps, you get a new location to go to or even a different icon background (the icons on the top and bottom are trees and houses, you can get new icon backgrounds, the trees and houses will change into apples for example then) to get those things, you need to raise 4 generations of characters which have all gotten their 4 tamatomo stamps, you don't need to raise those 4 characters in a row.
I hope this helps!