Nikki

Nikki is a character who first appeared in Swapnote as a guide for the user, a role she reprises in its successor, Swapdoodle. Although she initially appeared exclusively as a Mii, an illustrated design of her was later introduced.

StreetPass Mii Plaza
Nikki's Mii was distributed through StreetPass Mii Plaza, where she would appear in the player's plaza after downloading Swapnote from Nintendo eShop.

Super Mario Maker
Nikki appears as one of many unlockable costumes for use with the Mystery Mushroom, using sounds and music from Swapnote.

Nintendo Badge Arcade
In Nintendo Badge Arcade, many Swapnote-themed badges were released in the "Nikki & Friends" series, often including original artwork. When this series was introduced in December of 2015, a scenario played out where the Arcade Bunny showed the player a crude self portrait he was drawing as a present for a friend of his, revealed to be Nikki. When the Arcade Bunny answers the door, he receives a letter from Nikki stating she's knitting him a holiday sweater, shown to the player as a photo taken in AR Games through Mii Pics, and written on with Swapnote.

Super Smash Bros. Ultimate
Nikki appears as an Assist Trophy. After being summoned, she appears in the bottom-left corner of the screen, similarly to in her home series, and begins drawing. Her drawings are mostly hazards for her summoner's opponents, such as a fire-breathing monster or a Bullet Bill, but she can also draw a functioning Smash Ball.

Trivia

 * Nikki's birthday is December 21st, the date on which Swapnote released in Japan.