Mikan Tachibana is the main protagonist of Shin Atashinchi and serves as the atashi, or I, referenced in the series title. She is a seventeen-year-old second-year high school student attending Kusai High School, where she is a member of class 2C and part of the Teddy Bear Brigade. Her commute to school is long and stressful, and her school does not require uniforms. She lives in a three-bedroom apartment on the fifth floor with her mother, her father, and her younger brother Yuzuhiko.

Mikan is portrayed as an easy-going and relaxed girl who enjoys having fun. She presents a maiden-like, polite demeanor to strangers and classmates, but around her close friends, family, and fellow members of the Teddy Bear Club, she reveals a more tomboyish and uninhibited side. She is described as plain, timid, and a bit clumsy. She is also a heavy sleeper, which frequently makes her late for school, and she often falls asleep while doing homework, leading to incomplete assignments. She struggles with taking notes and generally gets by in her studies. She is a dreamer who frequently worries about her appearance, her weight, and her social standing at school.

Mikan has a goal of becoming a famous teddy bear designer or fashion designer. She taught herself how to sew buttons by observation at the age of three, and she enjoys creating original teddy bears, admiring expensive teddy bear toys, and participating in teddy bear acts with her club. Her interests also include karaoke and eating sweets, although she does not like chocolate cake. She once briefly wanted to become a breadmaker but lost interest.

Her role in the story is that of the primary viewpoint character, frequently providing a relatively normal perspective against which the antics of her eccentric family are measured. She often finds herself embarrassed by her mother's loud and frugal behavior, leading to frequent mother-daughter bickering. She is frustrated by her mother's strict rules regarding her allowance, her fashion choices, her curfew, and especially her simple boxed lunches, and she resents that her younger brother seems to get away with more. Despite their conflicts, her relationship with her mother is loving. She shares a relatable sibling rivalry with her younger brother Yuzuhiko. Her relationship with her father is less directly featured, though the family dynamic is central to the series.

Mikan's closest friends at school are Shimi-chan, whose last name is Shimizu, who is mature, calm, and provides practical advice on topics such as diets and dealing with parents, and Yukarin, who loves karaoke and introduces Mikan to cuter aspects of life. Mikan has a crush on her classmate Iwaki, a bishonen type. She wants to confess her feelings to him but is often thwarted by his best friend, Yoshioka, whose teasing gets under her skin. Yoshioka has attended the same schools as Mikan since elementary school, and people, including her mother, suggest she should date him. This creates misunderstandings, as Shimi-chan sometimes assumes Mikan is looking at Yoshioka when she is actually looking at Iwaki.

Mikan's development across the series focuses on her gradual maturation and her efforts to carve out her own identity separate from her unusual family. Over time, she learns to appreciate her mother's quirks even as she continues to strive for a more sophisticated and independent lifestyle. Her notable abilities include her skill in sewing and creating teddy bears, which she practices through her involvement in the Teddy Bear Brigade. She is also characterized by her resilience in navigating both her chaotic home life and the typical struggles of adolescence with a blend of anxiety and humor.