Katie Forester is a main character in the Yo-kai Watch anime, known as a friend and classmate of Nate Adams at Springdale Elementary School. She is an eleven-year-old girl with black eyes and brown hair typically tied in a ponytail with a signature pink ribbon. Her usual outfit consists of a cream-colored cardigan with teal trimmings and a small pink heart design, worn over a black and white striped top, paired with blue shorts, dark grey socks, and black trainers with white soles and laces. In the Japanese version of the series, she is known as Kodama Fumika, often called Fumi-chan.

Katie is portrayed as a kind, playful, and popular student who carries a certain charm that makes her well-liked by her peers. She is generally smart and cheerful, though she can sometimes worry about living up to her mother's expectations. One of her notable traits is being surprisingly brave and comfortable with heights, which contrasts with her otherwise ordinary demeanor. However, she tends to be quite oblivious when it comes to matters of romance, often missing or misinterpreting romantic cues directed at her. When affected by the influence of mischievous Yo-kai, she may retain her cheerful exterior while acting in unintentionally sharp or thoughtless ways, but she typically has no memory of these incidents afterward.

Within the story of the anime, Katie serves as a close friend to Nate and is frequently the target of various Yo-kai inspiriting, which creates situations that Nate must resolve. She is often seen alongside Nate's other friends, Bear and Eddie. A recurring dynamic in the series involves Nate attempting to impress her, though his efforts usually end in failure or awkwardness. Katie often refers to Nate as average, but she has also expressed that she feels very comfortable around him precisely because of that quality, indicating a genuine fondness for him. Her tendency to be inspirited by Yo-kai is a running gag, as these possessions often cause humorous misunderstandings or conflict between her and Nate.

In an alternate universe scenario depicted in the anime, Katie is shown as the wielder of the Yo-kai Watch instead of Nate. In this parallel world, her companion Yo-kai is Baddinyan rather than Jibanyan, and she takes on a more active role in dealing with Yo-kai incidents, such as when her mother is possessed by the Yo-kai Noway. Additionally, in the Yo-kai Watch Shadowside storyline, which takes place thirty years after the original series, Katie is depicted as an adult married to Nate Adams. As Katie Adams, she is the mother of two children: a daughter named Summer and a son named Tate. In this future timeline, she appears as a caring parent who shows concern for her children's well-being as they encounter supernatural events of their own.

In terms of abilities, Katie does not possess inherent supernatural powers in the main anime continuity. However, she is shown in certain storylines and alternate scenarios using a Yo-kai Watch, which she typically wears around her neck like a pendant, contrasting with Nate's wrist-worn model. In a notable episode, Katie briefly appears to have gained psychic abilities, but this is later revealed to be the result of being inspirited by the Yo-kai Espy. A defining characteristic of Katie is her resilience and the way she handles the unusual situations she finds herself in, whether due to Yo-kai mischief or the everyday challenges of school and friendships. While she may seem like an ordinary girl on the surface, her interactions with Nate and the supernatural world highlight her as a central figure in the series' social landscape and a key motivator for many of Nate's actions.