Michiru Saiki appears in the anime ReLIFE as a supporting character whose role is primarily shown through the memories of the protagonist, Arata Kaizaki. She was his senior colleague and mentor at the company where he worked after graduate school. Saiki had long brown hair that she typically kept in a ponytail. She was known for her approachable, warm, and nurturing personality, and she was well-liked by those around her. As a hardworking and talented employee, she took her responsibilities seriously and found joy in guiding new employees like Kaizaki, offering him advice on how to navigate office politics and project demands.

Beneath her composed exterior, Saiki was enduring severe workplace harassment from jealous veteran coworkers who resented her rapid promotions and sabotaged her work through tampered reports and unjust criticism. She internalized this suffering in order to maintain a professional demeanor and avoid escalating the toxic dynamic. When Kaizaki noticed her distress and tried to intervene, his efforts only intensified the harassment directed at her. Saiki rejected his help, discouraging him from getting involved, and continued to shield him from the full extent of her struggles. The persistent bullying ultimately drove her to take her own life by hanging.

Her suicide had a profound and lasting impact on Kaizaki, leaving him deeply traumatized. He quit his job soon afterward and struggled to find stable employment, becoming a NEET. This personal tragedy is the foundational trauma that makes him a candidate for the ReLIFE experiment. Within the story, Saiki serves as a poignant symbol of the destructive effects of toxic corporate culture in Japan, often referred to as black company practices. She does not undergo character development during the narrative because her fate is already sealed before the main events begin. Instead, her significance lies in how her memory and the grief she caused shape Kaizaki's motivations, his interactions with others, and his own journey toward healing and second chances. Her notable ability was her capacity to maintain a cheerful and supportive facade while carrying a heavy emotional burden, and her strong sense of responsibility as a mentor who prioritized the well-being of those under her guidance over her own safety.