Serena Rinnen was a thief who raided merchant caravans until her life changed dramatically. Her entire group was decimated when they attacked a caravan protected by a Ranker, forcing her to confront the fragility of human life. After this event, Headon approached her and chose her as a Regular, teleporting her to the Inner Tower to begin climbing.

Physically, she presented as a plain-looking woman with brown hair, brown eyes, and an ample bust. She typically wore a plain brown dress-like garment and carried a small knife. Upon entering the Tower, intense determination to gain power and a resolve to never feel helpless again marked her personality, driven by the trauma of her friends' deaths. She initially thrived in the competitive environment, attacking and dismembering rivals without hesitation during the first elimination test on the Second Floor and feeling thrilled by the sense of control it provided. She later teamed up with Phonsekal Laure and Ho.

Her development began during subsequent tests. During Lero-Ro's shinsu barrier test, she initially failed alongside Shibisu. Their competitive bickering turned into mutual encouragement, and they eventually forced their way through the barrier through sheer willpower, sharing a high-five afterward. This interaction marked a shift in her perspective. She started observing other Regulars, particularly Team A, and found herself cheering for them. Imagining using such individuals as stepping stones to ascend repulsed her, leading to deep self-disgust about her past actions and motivations.

In the Crown Game, she participated with Laure and Ho but surrendered after Anak ignited her weapon during their confrontation. Her internal conflict deepened during the Hide-and-Seek Test on Team B. Positioned with Ho per Androssi's plan, she openly confessed her changing feelings: her initial anger and thrill in killing had given way to camaraderie and regret. After Ho ran off during their conversation, Parakewl and Mauchi captured her. She then witnessed Ho taking Rachel hostage, Baam intervening, and Ho's subsequent suicide. This tragedy, coupled with Androssi's revelation of her manipulative intentions, led Serena to slap Androssi in outrage, though Androssi countered with criticism. Serena processed these events while sitting beside Ho's corpse and later attended his funeral.

Serena ultimately failed the Position Test. At the celebration party for successful Regulars, she chose to leave the Tower quietly. Shibisu intercepted her departure, questioning why she left without saying goodbye. Serena revealed her life felt full of regrets, but she resolved to "buckle down" and give life her all, hoping to find "one or two reasons to live" by doing so. When Shibisu asked if climbing the Tower wouldn’t provide that reason, she ignored the question. She gave Shibisu her cherished knife as a parting gift, instructing him, "Don’t you dare die," before smiling and walking away. This departure underscored her commitment to seeking purpose beyond the Tower's climb.

She exhibited no special powers or notable physical prowess. She struggled significantly with basic tests like Lero-Ro's shinsu barrier and relied heavily on stubborn determination and willpower to overcome challenges. Her combat skills appeared limited to basic hand-to-hand fighting and knife use, consistent with her background as a thief.