Tomoki Sakurai is a teenage boy in Sorami City, initially craving a peaceful life. Recurring dreams of an angel since childhood hint at a connection to the sky realm of Synapse. Abandoned by his parents at age ten to travel the world, he was raised by his grandfather. This grandfather instilled a preoccupation with eroticism through constant exposure to perverse quotes and behaviors. Intense social isolation followed an incident where friends were injured during an expedition to a bird's nest Tomoki discovered. Blamed by the other children's parents, he was shunned until Sohara Mitsuki moved next door. Both lonely, they formed a close bond; Tomoki taught her karate and citrus cultivation.

Tomoki embodies contradictions. He openly displays lecherous tendencies, frequently groping or peeping on females—traits he attributes to his upbringing. Yet, he simultaneously exhibits exceptional kindness and sincerity. This duality shapes his interactions with Angeloids, mechanical beings from Synapse who enter his life. While lamenting how they disrupt his desired tranquility, he genuinely cares for their autonomy, urging them to embrace freedom from servitude and make independent choices. He forms deep emotional bonds, eventually admitting they feel like family. Despite his perverse actions toward most females, he entirely refrains from such behavior with the primary Angeloid, Ikaros, demonstrating unconscious respect or deeper affection.

His relationship with Ikaros forms a central emotional arc. After she crash-lands near him, she imprints on him as her master, symbolized by a chain linking her collar to his hand. Tomoki struggles to understand her emotional detachment. In a pivotal moment in "Eternal My Master," frustrated by her perceived lack of emotion, he breaks the imprinting chain to free her. This act devastates Ikaros, who interprets it as rejection. Witnessing her despair, Tomoki realizes his mistake and the depth of her feelings. He reaccepts the master bond, acknowledging her desire to stay with him. Their relationship culminates in a kiss and mutual love confession, highlighted during her sacrificial act to carry him to Synapse, where she burns up but is later resurrected.

Tomoki's romantic entanglements form a complex "love pentagon" involving Angeloids Nymph, Astraea, and Chaos, plus childhood friend Sohara. He harbors feelings for all but remains largely oblivious to their affections. He develops a significant connection with Hiyori Kazane, a human girl secretly linked to Synapse. In "The Angeloid of Clockwork," Hiyori joins Tomoki's club and confesses her love. After she dies in an accident, Synapse erases her existence from everyone's memories—except Tomoki's. He retains fragmented memories, causing intense distress. When Hiyori reappears as a brainwashed Angeloid (Zeta) ordered to kill him, Tomoki risks his life to reach her. Unable to prevent her self-destruction, his intervention allows her to regain her senses for a final, heartfelt farewell. A post-credits scene implies her potential return.

Tomoki demonstrates unexpected resilience and leadership. He exhibits near-superhuman tolerance for physical punishment, particularly to his groin. Crucially, he repeatedly displays selflessness, intervening to protect others regardless of personal danger. This includes confronting powerful Angeloids to shield Nymph, physically attacking the ruler of Synapse, Minos, and vowing to save Chaos despite her instability. In the manga's climax, facing global annihilation, he uses the last universal wish to reset reality, sacrificing his own existence to resurrect fallen friends and restore the world, marking his evolution from self-proclaimed pacifist to decisive protector.

In the spin-off "Yūryō Bukken Mō Dame Sō," set years later, Tomoki is twenty-six and unemployed. He maintains a connection to Synapse events, interacting online with characters like Yurara Tategami, whom he supplies with cosplay outfits, continuing his interest in provocative attire.