Rui Ayase is a junior high school student and Ran Isozaki's childhood friend. He exhibits a calm, level-headed personality and frequently demonstrates intelligence by deducing critical clues within the mysteries they encounter. Rui remains perceptive to Ran's emotional state, recognizing subtle signs of distress like her distinctive, brief laughing fits. His interest in art leads him to join the school's art club.

Lacking inherent psychic abilities like telepathy or telekinesis, Rui possesses the unique supernatural power to nullify and amplify the psychic powers of others, such as Ran's telepathy or Midori Naha's similar abilities. This power proves instrumental during conflicts, especially against adversaries with supernatural capabilities. He supports Ran unconditionally upon learning of her psychic powers, expressing fondness for her true self. Their relationship carries romantic undertones, with mutual affection acknowledged between them.

His interactions with Midori Naha involve frequent clashes due to her abrasive demeanor and ego, though they collaborate when necessary to resolve supernatural incidents. Rui prioritizes protecting those close to him, occasionally acting decisively when Ran or others face danger.

Physically, Rui has short brown hair, brown eyes, and a tanned complexion. Within the group, his role centers on providing emotional stability and logical reasoning, complementing Ran's intuition and Midori's strategic approach. No significant character evolution or background expansion beyond these dynamics is depicted across the series.