Nikki

Nikki is the Mascot Mii for the Swapnote Series, and has made several appearances in other games since then.

Swapnote / Nintendo Letter Box
You can find all notes sent by Nikki here

Nikki's first appearance, Swapnote was a game where the player can draw notes and send them to their friends online.

Nikki was a guide to the player and would teach them how to do things in the game. The player could enable the ability to receive messages from her, and if enabled, the player would receive messages of updates on upcoming Nintendo news and simply sending them good morning and words of encouragement.

When the Swapnote service was shut down in October 2013, Nikki sent the player a message that they were "going away for a while", wished the player good luck, and left.

Nintendo News
Nikki appeared in the Nintendo News section of the Nintendo website. She was redesigned and joined by a panda and yellow cat.

Nikki's Travel Quiz
Nikki's Travel Quiz was a Japanese Club Nintendo exclusive game, featuring Nikki as the star.

The Player travels to different locales in Japan and is tasked with answering quiz questions.

Swapdoodle
You can find all notes sent by Nikki here

Swapdoodle is the sequel to Swapnote. In it, Nikki appears in the same role as in Swapnote. In the first letter she sends to the player she states it's been a while since she last saw them, referencing the shutdown of Swapnote.

Nintendo Badge Arcade
Nikki has many badges in Nintendo Badge Arcade.

Super Mario Maker
In Super Mario Maker, you can collect a Mystery Mushroom, which allows you to transform into Nikki. To place Mystery Mushrooms with Nikki in your levels, you need to clear the "100 Mario Challenge" on Normal difficulty or higher. This doesn't return in Super Mario Maker for Nintendo 3DS or Super Mario Maker 2.

Super Smash Bros. Ultimate
Nikki appears as an Assist Trophy in Super Smash Bros. Ultimate. She draws pictures which then come to life and appear on the stage. She uses a model that matches how she draws herself, instead of her Mii.

Gallery
Find more images here