Aki Sawada is a character from Kamen Rider 555, initially appearing as a troubled young man who transforms into the Spider Orphnoch. His past is rooted in tragedy; he was once a kind-hearted student at the Ryusei School, a group of orphaned children trained in martial arts, where he shared a close bond with a girl named Mari Sonoda. During a deadly reunion of the school, he and his friends were killed by the powerful Orphnoch known as Kitazaki. Sawada was later resurrected through an experiment by the Smart Brain corporation, which implanted him with the "Orphnoch's mark" and forcibly turned him into an artificial Orphnoch.

As a person, Sawada is defined by his inner turmoil and the violent conflict between his remaining human emotions and his new monstrous instincts. Before his change, he was remembered as a gentle individual, but as the Spider Orphnoch, he becomes a ruthless killer. He is frequently seen wearing a hat and a headset, listening to hip-hop music at a high volume. He has a distinctive ritual before an attack, where he folds a piece of paper into an origami animal, lights it on fire with a match, and watches it burn as he carries out his violent acts, with the flame extinguishing at the moment his final victim falls. This habit symbolizes his desire to burn away his past and his humanity.

Sawada's primary motivation is his obsessive love for Mari Sonoda, a feeling that has persisted since their childhood at the Ryusei School. However, rather than seeking to protect her, his twisted logic leads him to believe that he must kill her. He thinks that by destroying the person he cares for most, he can completely sever his ties to his human heart and fully embrace his identity as an Orphnoch. In his quest to prove his worth and abandon his humanity, he joins Lucky Clover, an elite group of powerful Orphnochs serving Smart Brain. He earns his place by killing Saya Kimura to obtain the Delta Gear, a powerful Rider belt. However, his membership is short-lived. His fixation on Mari is seen as a weakness, and he is branded a traitor and expelled by the group's leader, Kyoji Murakami, after he gives the Delta Gear to Mari and her friends.

His role in the story is that of a tragic antagonist and a dark mirror to the protagonist, Takumi Inui. Both are Orphnochs struggling with their identity, but while Takumi strives to protect humanity, Sawada actively fights against his own compassion. Sawada is intensely jealous of Takumi, who can live as an Orphnoch without losing his humanity, and this jealousy fuels their confrontations. The key relationship in his life is with Mari, who desperately hopes to save him and return him to the kind person he once was. He also has a significant, though negative, relationship with Masato Kusaka, another Rider who sees Sawada as irredeemable and refuses to forgive his sins.

Throughout his appearance, Sawada undergoes a tragic development. Because he is an artificially created Orphnoch, his body is unstable and begins to crumble into ash. As his body deteriorates, he begins to waver in his resolve. Seeing the kindness that Mari and Takumi still show him causes him to feel regret. In a final act of redemption, he knocks Takumi unconscious to prevent him from facing a suicidal battle against Lucky Clover and goes to fight in his place. Already weakened, he is brutally defeated by the other three members of Lucky Clover. When Kusaka arrives, Sawada looks to him for help, but Kusaka refuses, stating that no matter how his heart has changed, his crimes cannot be forgiven. Mortally wounded, he crawls away and is found by Mari and Takumi. He dies in the arms of the woman he loved, finally regaining his human heart at the very end of his life.

As the Spider Orphnoch, Sawada possesses a range of spider-like abilities. He can crawl on walls and vertical surfaces with ease. His joints, particularly in his shoulders and knees, are bellows-like and can be extended, allowing him to stretch his limbs for increased reach in combat. A single large eye on his face serves as a sensitive sensor, granting him superb accuracy. His primary weapon is the Hapo Shuriken, a large, wheel-like, eight-spiked shuriken that he can use as a melee weapon, throw for long-range attacks, or use to inject a paralyzing poison into his opponents. He can also extend tentacles or spider-like hairs from his body to impale enemies or inject Orphnoch energy. Despite these formidable powers, his artificial nature makes him incomplete. His fighting strength is considerable but is ultimately no match for the highest-ranking Orphnochs like the Rose Orphnoch or Kitazaki, and his physical form is fated to decay.