Eterna Rinebow is a water-element magic user from the anime Roll Over and Die, noted for possessing extensive knowledge in the field of medicine. Despite her outward appearance as a young girl, she is revealed to be considerably older than other characters, with sources indicating she is at least sixty years old. This youthful appearance is a direct result of her past; as a child, she was selected as a test subject by the Church of Origin, and the experiments conducted on her caused her aging process to slow dramatically. During this period, she was adopted by the Rinebow family, who taught her about medical herbs, and the three came to love each other as a real family. When the experiment was eventually shut down, her parents helped her escape, and she lived in the mountains for over fifty years, continuing to age very little.

Eterna generally carries herself with a stoic demeanor and is not afraid to speak her mind, though she possesses a deeper level of care for her companions than she initially lets on. She has a laid-back personality and a pace that is all her own, but she is deeply loyal to her friends. For example, she willingly helped cure Milkit of a severe poison and acted as her protector. When she learns that fellow former party member Jean Inteige lied to Flum Apricot, claiming Eterna approved of her being sold into slavery, she cannot hide her shock and disgust, and expresses a strong desire for revenge. After becoming roommates with Flum and Milkit, she demonstrates a playful and teasing side, particularly enjoying teasing them about the nature of their close relationship.

Her motivations are deeply tied to her found family. At one point, she was tempted to leave her new life behind to reunite with the spirits of her deceased parents, but she ultimately decided that the life she had built with her roommates was far more important. She also joined the Hero's Party partly to learn what became of her parents. Her role in the story becomes significant after she leaves the Hero's Party. She ends up living with Flum and Milkit, providing them with a place to stay, sharing her medical and magical knowledge, and acting as a powerful protector for them against threats like the corrupt noble Dein.

Her key relationships are central to her character. She was one of the only members of the original Hero's Party, alongside Gadhio Lathcutt, to consistently treat Flum well, and in turn, she considered Flum a friend. Despite not seeing herself as a teacher, she taught Flum about herbs and magic, lessons that later prove invaluable. Her relationship with Milkit is also protective; she respects Flum's wish to keep Milkit safe but has no qualms about eliminating those who would harm her. She is accompanied by two round bird creatures that assist with her magic; these creatures often seem to reflect her opinions, reacting positively to Flum and with anger towards Jean.

Eterna develops by choosing loyalty and a peaceful life over the past. While she was tempted by the chance to be with her parents again, she decides her new life with Flum and Milkit is more important, demonstrating her growth from a solitary figure to someone who values her chosen family. Her notable abilities include her proficiency with water-element magic and her exceptional knowledge of medicine, which allows her to identify poisons, create cures, and treat wounds. She is also a capable combatant, able to casually defeat multiple armed men with her magic. Living for over fifty years in the mountains, she put the knowledge her adoptive parents gave her to practical use, becoming known as the Timeless Witch.