Natsumi Hayamiya is a supporting character in the Birdy the Mighty Decode series, appearing in the television seasons and the bridging OVA The Cipher. She is a classmate and close personal friend of Tsutomu Senkawa, the protagonist who shares a body with the alien Federation agent Birdy Cephon Altera. Natsumi is depicted as a typical teenage girl with short brown hair, red eyes, and often seen in her school uniform. Her primary role is that of a loyal friend within Tsutomu’s everyday life, providing a normal social anchor amid the increasingly extraordinary events surrounding him. She is generally cheerful and supportive, though her personality is not explored in great depth. Natsumi’s motivations revolve around her friendship with Tsutomu and her concern for his well-being, especially when his behavior becomes erratic due to his dual existence with Birdy. In The Cipher, she is part of the group that accompanies Tsutomu and Sayaka Nakasugi during a trip to Kobe, where a Federation marionette complicates Birdy’s mission. Natsumi does not possess any special abilities; she remains a regular human who is largely unaware of the alien conflict, serving as a grounding presence in the series’ science-fiction context. Her development is subtle, as she remains a steadfast friend throughout the events of the Decode timeline.