Sui is a youth hosting dual consciousness—her own soul and that of her deceased twin brother Sōta, who intermittently controls their shared body. Initially shy and averse to violence, Sui avoids confrontation, wrestling with the ethics of survival within a lethal game. In stark contrast, Sōta exhibits aggressive boldness, advocating unrestrained combat and lethal solutions. Their opposing personas fuel speculation that Sōta may be a dissociative identity or psychological coping mechanism rather than a separate entity.

After a five-year timeskip, Sui embraces pragmatism, demonstrating ruthless tactics like suffocating captives or manipulating bodily fluids to simulate drowning. Hidden beneath this hardened exterior lie unspoken feelings for clanmate Ryūji, subtly recognized by Sōta but left unacknowledged.

The twins’ abilities form strategic complements: Sui’s Pollux Light commands precise water manipulation—controlling atmospheric moisture and bodily fluids for offensive strikes or medical aid, such as extracting bullets. Sōta’s Castor Light employs cryokinesis, freezing water-based substances to craft barriers or immobilize foes. Post-timeskip enhancements allow Sui to draw moisture directly from the air, amplifying tactical flexibility.

Physically, Sui evolves from short pale blue hair and a white hooded coat to longer mid-back locks paired with darker, utilitarian attire. Their sustained androgynous appearance post-puberty sparks theories about Sōta’s influence on physiological development.

Key narrative moments—like leveraging water manipulation during the Treasure Hunt Game to corner opponents and investigating the Out of Range Village incident—mark Sui’s shift from passivity to decisive action. Evolving from guilt over failing to protect Ryūji to deeper, unspoken emotional ties, their relationship mirrors Sui’s growth amid crises, balancing vulnerability and resolve.