Ruby mentors Momona, an ordinary middle school girl transported to Jewel Land to become a candidate for the next Lady Jewel. Acting as Momona's partner, Ruby guides her through the tasks and challenges required to earn the title, offering magical aid and emotional support despite her own shortcomings. This mentorship role arises from Ruby's position within the Jewelpet hierarchy, though it is later revealed Ruby did not personally choose Momona; an unknown individual brought Momona to the Jewel Palace during her cousin's wedding.

Ruby exhibits extreme air-headedness and a consistent lack of situational awareness. She struggles to grasp social cues, frequently misinterpreting insults as compliments. This trait impacts her mentorship, as she often fails to provide Momona with accurate or practical guidance. Her decision-making is impulsive, especially during stressful situations, leading to hasty actions without considering consequences. Despite these flaws, Ruby maintains unwavering optimism and loyalty. She remains dedicated to Momona's growth and persists in her mentorship through setbacks.

Ruby's magical abilities remain unreliable, consistent with her franchise portrayal. She wields a Jewel Pod and Magical Letterpen alongside Momona to perform spells during tests and challenges. However, her magic retains a tendency for failure, often causing unintended outcomes or explosions. This ineptitude creates comedic situations and occasional obstacles for Momona's tasks. Ruby also demonstrates physical resilience, enduring frequent mishaps and injuries with quick recovery.

Ruby's relationship with Momona evolves beyond mentorship into friendship. She provides emotional encouragement when Momona faces self-doubt, particularly regarding her feelings for Prince Cayenne and her rivalry with Lillian. Ruby's persistent positivity helps Momona navigate complex social dynamics within the Jewel Palace, including Lillian's initial jealousy and the revelation about Momona's mysterious selection. Their partnership strengthens throughout the series, culminating in mutual support during the final selection process.

Background elements from the broader franchise contextualize Ruby's presence. As a recurring mascot character representing luck or courage, she maintains her iconic appearance: a white Japanese hare with pink inner ears, ruby jewel eyes, a pink pearl necklace featuring cherry charms, and a cherry blossom hair accessory with a star center. Her role in "Lady Jewelpet" exists within this continuity, where she consistently partners with human protagonists across different storylines while retaining core traits like magical unreliability and cheerful determination.