Anna Kobayakawa is a central character in the anime Rainbow Days, known as the female lead and the primary love interest of the main protagonist, Natsuki Hashiba. She is a high school student whose life changes after a chance encounter on Christmas Eve. While working a part-time job dressed in a Santa Claus costume, she came across a boy crying in the street and offered him a tissue. That boy was Natsuki, who had just been dumped by his girlfriend. Touched by her simple act of kindness, he gave her a scarf he had been planning to give to his ex. This brief meeting sparked Natsuki's deep affection for her, a feeling that grows when he later discovers they attend the same school.

In terms of appearance, Anna is a pretty teenage girl with straight, neck-length peach-blonde hair and green eyes. Her outward demeanor is often calm, quiet, and reserved, which initially leads others to perceive her as gloomy, mature, or hard to read. She is not very expressive and rarely shows her emotions on her face, a trait that makes her a contrast to the more boisterous main characters. However, this reserved nature masks a deeply kind and caring personality. Anna is thoughtful, independent, and hardworking, taking on several part-time jobs. She is also honest and speaks earnestly, and despite her quietness, she has no trouble socializing with her classmates when the situation calls for it.

Anna's motivations are primarily tied to her personal growth and her evolving understanding of romance. At the start of the story, she is inexperienced in matters of the heart and is not accustomed to interacting with boys. Her world largely revolved around her close friend, Mari Tsutsui, who was one of her only friends. As Natsuki persistently and gently tries to get closer to her, Anna slowly begins to navigate new feelings of confusion, shyness, and eventual affection. A key moment in her development occurs during a school festival when Natsuki has to cancel their plans, causing Anna to feel an unfamiliar sense of disappointment and jealousy upon seeing him with other girls. This event helps her realize she has deeper feelings for him than she initially thought. Her romantic awakening culminates when Natsuki kisses her during a Christmas date, an event she initially cannot remember but which prompts her to finally acknowledge that she has fallen in love with him.

Anna's role in the story is central as the catalyst for much of the plot. Her relationship with Natsuki is the main romantic focus of the series. Her key relationships include her strong, protective friendship with Mari Tsutsui, who is often wary of the boys trying to get close to Anna. As the story progresses, Anna becomes a more open person, growing close not only to Natsuki but also to his group of friends: Tomoya Matsunaga, Keiichi Katakura, and Tsuyoshi Naoe. Through these interactions, she transitions from a solitary girl to an integral part of a larger, warm-hearted social circle, becoming more confident and showing her emotions more freely as she experiences friendship and first love. Among her notable abilities, Anna is shown to be a diligent and hardworking individual who can balance her school life with multiple part-time jobs.