Keith Claes is the adopted younger brother of Katarina Claes and a central figure in the story. He is a handsome young man with wavy flaxen hair and blue eyes, standing around 180 centimeters tall. Originally born as Keith Coleman, he is the illegitimate son of Viscount Coleman and a commoner woman. His early life was marked by profound neglect and abuse from his biological family, including his half-brothers, which caused him to develop powerful earth magic that he could not fully control. When a bullying incident triggered his magic and injured his tormentors, his fearful family isolated him until Duke Luigi Claes, recognizing his magical potential, adopted him at the age of eight.

In the original otome game Fortune Lover, Keith’s adoption into the Claes household does not lead to happiness. His adoptive mother, Duchess Millidiana Claes, falsely believes he is her husband’s illegitimate child and treats him coldly, while his adoptive sister Katarina bullies him. As a result, he grows up lonely and becomes a playboy, seeking emotional fulfillment through fleeting romantic relationships. In his game routes, the heroine’s kindness heals his loneliness, leading to either Katarina’s exile or, in a bad ending, her death at his hands.

However, the present timeline is altered drastically by Katarina, who has regained memories of a past life and actively works to avoid her doom. Instead of bullying him, she treats Keith with consistent kindness, plays with him daily, and clears up the misunderstanding about his parentage with Duchess Millidiana. Consequently, Keith does not grow up lonely. He becomes a hardworking, intelligent, and gentlemanly young man, though Katarina worries that her instruction to treat girls kindly has inadvertently turned him into a charming ladies' man. Beneath this courteous exterior, he is genuinely shy around other women and easily flustered by physical affection, especially from Katarina herself. He is deeply aware of his good looks and charm, and will use them when necessary, particularly in matters involving his sister.

His relationship with Katarina evolves from deep familial loyalty into romantic love. He becomes fiercely protective of her, acting as her minder to curb her impulsive tendencies and viewing her suitors, particularly Prince Geordo, as rivals. For years, he keeps his true feelings hidden, but a significant turning point occurs when he is kidnapped by his biological brother Thomas and dark magic users for an experiment. After Katarina rescues him, he confesses his love for her, finally making her aware of his romantic intentions. This confession marks a shift in his character, making him more openly assertive about his feelings.

In the spin-off Verge of Doom, where Katarina regains her past life memories later, Keith initially despises her for the abuse she represents in the original game. It is only through her sincere efforts to atone that he gradually accepts her, forging a genuine sibling bond that helps him reconcile with his own neglected childhood.

Keith possesses exceptionally powerful earth magic, considered among the strongest of his generation. His signature ability is creating and controlling large golems, which he can deploy for combat or other purposes. He is also highly intelligent and excels in his studies at the magic academy. His magical strength is such that his birth family feared him, and it is the primary reason for his adoption by the Claes family. Beyond his relationship with Katarina, he shares a respectful dynamic with the family's maid, Anne Sherry, as they work together to watch over Katarina. Over time, Duchess Millidiana comes to regard him as a favored son, trusting him to uphold the family's reputation.