A 17-year old high school student, and is the intended "main protagonist" of the series. (The "atashi" (me) in the title.) She goes to school in a public high school, which unfortunately is a long, stressful commute away. She is portrayed as an easy-going, relaxed girl who likes to have fun. Often very maiden-like, but other times very tomboyish, she goes through every day with some funny thing happening.
Her best friends in high school consist of the "adult"-like Shimi-chan (her last name Shimizu), who always stays calm and collected, and always has good advice from diets to dealing with parents; and Yukarin (her first name Yuka), who loves karaoke and introduces Mikan to the "cuter" things in life.
Mikan has a crush on the bishōnen-type Iwaki. Although she wants to confess her feelings to him, she is often foiled by his best friend, Yoshioka, whose wisecracks get under Mikan's skin.
What's worse is that Mikan and Yoshioka have attended the same academies since Elementary School, and people (including her own Mother) persuade her to date Yoshioka. At times, when she looks in the direction of Iwaki and Yoshioka, Shimi-chan gets the wrong idea and assumes Mikan is staring at Yoshioka, not Iwaki.
She "gets-by" in school, is horrible at taking notes, and is a heavy sleeper, which unfortunately makes her late to school. She also has a habit of falling asleep through homework assignments, causing her to have no choice but to submit incomplete assignments.
Her interests include karaoke, eating sweets (although she dislikes chocolate cake), and admiring/creating teddy bears.
She is part of her high school's "Bear Brigade" (composed of the wealthy, fashionable Rio; the geeky-yet-romantically-involved Nitta; and best friends Kajii and Asada), who create original teddy bears, perform teddy bear acts, and admire expensive teddy bear toys.
Her goal in life is to become a famous teddy bear and/or fashion designer. According to her mother, she taught herself how to sew on buttons (through observation) at the age of 3. She once fancied being a breadmaker, but lost interest soon after.
She is often bogged down by her Mother's rules, which inhibit her allowance, her fashions, her curfew, and mostly—her simple boxed lunches. She is frustrated that her Mother lets her younger brother get away with some rules, however, she often (and cleverly) makes her case.