Mother Tachibana, often called Okaa-san Tachibana, is a Japanese housewife residing in Tokyo with her husband and their children, Mikan and Yuzuhiko. Born in Ōita Prefecture with the childhood nickname "Banban," she grew up on her family's farm. There, she frequently played with insects and helped with farm work under her mother's strict supervision and harsh discipline for minor mistakes. In her youth, she harbored romantic feelings for a childhood friend, hoping to marry him, but ultimately entered an arranged marriage (Omiai) with her future husband. After marriage, the couple ran an onsen in their hometown before moving to Tokyo due to her husband's job transfer.

She stands 165 cm (5'5") tall, aging from 43 to 45 throughout the series. Her appearance features brown hair and prominent lips. She typically wears casual home attire: solid-colored T-shirts (often yellow) with pinkish skirts and aprons, though this outfit appears less often later. Her hairstyle evolved from long curls in her youth and early motherhood to shorter hair as she aged, with her features gradually resembling her mother's more closely.

Her personality blends traditional Japanese housewife values with comedic contradictions. She takes pride in maintaining a clean home, balanced budget, and preparing frugal yet palatable meals, often multitasking household chores, though clumsiness sometimes causes mishaps. She pursues supermarket bargains with intense enthusiasm, even physically competing for discounted items. Conversely, she displays laziness through habitual afternoon naps, watching television while eating senbei rice crackers, and neglecting shopping or cooking on rainy days—sometimes resorting to aged ingredients. She holds traditional views, particularly prohibiting daughter Mikan from ear piercing, late-night karaoke, and fashionable clothing while emphasizing housework over academics, yet shows leniency toward son Yuzuhiko. Her social life centers on afternoon teas and shopping with friends Mrs. Mizushima and Mrs. Toyama, where they often envy and try to emulate the affluent lifestyle of Mrs. Misumi on a budget. Earlier portrayals depicted her as more religious and stern, but her personality softened over time, with the religious aspect diminishing. She occasionally feigns knowledge about unfamiliar topics, leading to awkward situations. Her hobbies include calligraphy (shodou), cooking, and eating chikuwa fish paste.

Significant development occurs across media. In the film "Eiga Atashin'chi", a lightning strike during a storm causes her to swap bodies with Mikan. Navigating each other's lives, she attends school in Mikan's body, facing social embarrassment and inadvertently embarrassing Mikan while confronting generational differences. During Mikan's school trip to Kyoto—which she missed in her own youth due to finances—she participates in Mikan's body with help from Mikan's friend Shimi-chan, gaining new appreciation for the experience. The body-swap culminates in an attempt to reverse it using electricity during another storm.

Her background is further explored at a hometown class reunion, where Mikan (in her mother's body) reads a youthful letter revealing her past affection for Tsukioka Shuzo. Meeting Tsukioka, he shares nostalgic memories of their tree-climbing and offers perspective on appreciating family despite conflicts, resonating deeply with both. This reinforces her connection to her origins and highlights emotional growth regarding familial bonds.

Comedic situations reveal character aspects, like an outing with friends to a French restaurant where attempts to appear cultured devolve into uncultured comparisons of gourmet ingredients to common foods like brown tea and chocolate, causing Mrs. Misumi's dismay. These interactions underscore her middle-class sensibilities and contrast with her affluent friend.