Nikki is the mascot Mii in Swapnote who gives you tips and rewards every now and then. When you download the Swapnote application from Nintendo eShop, Nikki is added to your Mii collection in StreetPass Mii Plaza.
Her first and major appearance is in the 3DS application, Swapnote. Here she would instruct players how to use the feature, and after that players can chose weather or not to receive notes from her in the future. If players selected Yes, Nikki would continue to send notes to the player giving them updates on upcoming Nintendo news, as well as simply sending notes wishing them a good day, or sending words of encouragement.
After Swapnote had its online feature removed, Nikki sent a farewell note saying she was going away for a while. She wished the player luck, and then would leave.
She now appears in the Nintendo News section of Nintendo's website. She is also shown having gained two new assistants; a yellow cat and a panda.
Nikki is also the star of her own video game called Nikki’s Travel Quiz. Originally given out to Club Nintendo members who have had enough points to purchase it from the official site, it is a quiz game that features Nikki, her panda, and her cat.
Players travel to different parts of Japan, and Nikki will quiz players on certain locations, buildings, and peoples from said location. Nikki's Panda is capable of giving players hints if they are struggling to answer certain questions. Nikki will often times dressed up in different outfits, such as a sailor uniform, or a kimono, when talking about a particular subject or while visiting a specific region in japan.
With Club Nintendo having been taken down, there is currently no way to obtain the game.
Nikki also appears as an unlockable Costume Mario in Super Mario Maker, based on her redesign from Nintendo News and Nikki's Travel Quiz.
On December 7th, 2015, Baito shows the player that he's drawing a (humorously crude) picture of himself as a present for a friend of his. After guessing who this friend is, he confirms that this friend of his is Nikki, explaining that she was the face of Swapnote, "back in the day." After showing that Swapnote badges are now available in the arcade, he goes to answer the door, receiving a letter from Nikki showing that she's knitting a holiday sweater for him.
Nikki makes an unofficial appearance in WayForward's multiplatform game Shantae: Half-Genie Hero. As part of being a backer for the Kickstarter of Half-Genie Hero, those who donate enough money are allowed to design an enemy type that will appear in the game.
Designed by Backer Roger C, Ikki is a direct reference to Nintendo's Nikki. Ikki is a spirit who is seen wearing a green sweater, based on Nikki's Mii as she appeared in Swapnote. She sports Nikki's hair style, upper body proportions, and even her signature glasses.
As an enemy Ikki hides in treasure chests and may randomly appear when opened. From there Ikki will phase in and out, turning invisible and then re-appearing in a random place in the environment, and toss projectile weapons at players. This is the time to strike and make Ikki vanish, until she randomly appears when another chest is opened.
Nikki appears in Swapdoodle, a successor of Swapnote. In the first letter she sends to the player, she says that it's been a while since they saw her. Her role here is similar to her appearances in Swapnote and Art Academy: Lessons for Everyone!.
Nikki appears as an Assist Trophy in Super Smash Bros. Ultimate, She uses the design she uses when drawing herself as opposed to her Mii appearance.
She also makes an appearance as a spirit, which can be obtained in a battle against a Mii Gunner, Mii Brawler and a Mii Swordfighter.
Nikki stands out for having fair skin. She has a brownish-gold cut-bob with her bangs styled to the left and has brown eyes worn in red reading glasses. She is often seen wearing light green, and is short in height.
As of her re-design, Nikki is shown getting a white button-up undershirt, with a light green jacket. She now wears a black knee length skirt, with black stockings and shoes. She still sports her signature red glasses.
Nikki could be a Japanese-American, as her name, while definitely Japanese, also fits as American; her name in Japanese means "diary" (日記, にっき, Nikki) and in America, her name could be short for Nicole as the shortened variation, which would fit as the name Nikki was invented in America as a shortened variation of Nichole. The name Nicole is of french origin, and it means "Victory Of The People". Nikki's name is officially written as ニッキー in Japanese.
She seems to be even-tempered, always bouncy and cheerful, calm and determined. She tries makes you feel like there is always someone there for you. She sent you letters regularly which contain important information and updates. She likes to spend a lot of her time reading, writing and spelling at school, but isn't very good at something that she hasn't done before.