Tatsuya Saeki (冴木 達也 Saeki Tatsuya?) is a character in the Vivy: Fluorite Eye's Song series. He is a former Toak member, the first human married to an AI, Grace. During the singularity event prevention timeline, Grace was rewritten as the master of the Metal Float island so Saeki assisted Vivy to free her from it by destroying the Metal Island.
Being abandoned as a child, Tatsuya is mentally wounded from this fact and is unable to accept it. However upon usually staying in contact with Grace, a nurse AI of the hospital he is in, he gradually heals and comes to believe that AIs are the same as humans, the fact that their actions prove they do have a heart and what he experienced with Grace convince him to pursue the Human Rights for AI.
In the "official" history, in order to settle down the mistrust of human towards AI after the "Sunrise Fall" event, Tatsuya and Grace married and this is known as the first marriage serving as the bridge between human and AI. However, thanks to the heroic performance of Estella in the altered timeline, humans have regarded her as an ideal example that every AI ever created should be. The event earned the trust of human to establish an unmanned AI facility without rest, thus the creation of Metal Float. But in order to manage the huge tasks on the island, it requires an AI to fulfill the duty, and Grace was chosen to become the core - the Mother Computer, of the Metal Float.
Tatsuya joined Toak because they would help him in his objective of shutting down the island, but soon he realized they held a belief that is very much different than what he'd imagined, and parted ways with them. Later on he is pursued by Toak, who is after his shut-down program. He is saved by Diva. Tatsuya realizes who she is and asks for her help in this mission. Diva and Matsumoto agree to his plan but leave him behind as they cannot ensure his safety during the operation. After meeting again on the island, Tatsuya reveals his true objective to save Grace, or rather, rescue her data, since retrieving her physically from the core is impossible. He even goes to the extent of showing Diva what he believed is Grace's singing of Diva's song for years after she has become the core, trying to convince her to save her sister. However, Diva refuses after realizing what she heard is simply tone data left behind, and says that she's going to destroy Grace. Unable to see through her choice, Tatsuya points his gun and try to stop her but fails. While Diva was struggling with the rogue AIs of the island, he finally accepts the truth and sends her the data to the core with every pain in his heart, leading her to destroying Grace and shutting down the island. After the event, Tatsuya meets Diva in the church that he planned to hold his and Grace's wedding, and says that just like her mission to make everyone happy with singing through songs, Grace was the song of his life. Feeling that he has been robbed of everything in life, from his family to the person he now holds dearest, Tatsuya shoots himself, unexpectedly forcing Diva's system to freeze as she commits a fatal error.
- The name Tatsuya means "achieve" (達) (tatsu) and "to be, also" (也) (ya).
- Tatsuya's surname Saeki means "be clear, serene, cold, skillful" (冴) (sae) and "tree, wood" (木) (ki).