Lock, originally the fairy Kuroro from Hope Kingdom, was brainwashed by Dys Dark after Dyspear attacked him and imprisoned his fairy friends. This led to his possession by a sentient dark coat, transforming him into Lock, one of Dys Dark's commanders and a member of the Three Musketeers.

As Lock, he manifested as a young boy with obscured eyes beneath burnt orange hair, adorned with a demonic-eyed hood featuring horns, wings, and a central padlock. His attire consisted of a dark yellow-and-red coat, orange shorts, sneakers, and gray-and-black-striped sleeves and leggings. He often carried a handheld video game. Lock displayed a cold, manipulative personality, deriving pleasure from inflicting despair. He trapped victims' dreams to create Zetsuborg monsters using the phrase "Lock your Dream!". His abilities included fire projection and shadow manipulation.

Following Dyspear's retreat to heal, Lock assumed command of Dys Dark and aged into an adult form. This adult version wore a black-red-yellow coat, a black onesie with a red zipper, and redesigned footwear. His powers intensified, allowing him to create stronger Zetsuborgs via kicks instead of finger snaps. Exploiting his leadership position, Lock orchestrated the theft of the Cures' Dress Up Keys to fuel his ambition of overthrowing Dyspear. He converted the stolen dream energy into a giant castle Zetsuborg, declaring his intent to become the "King of Despair".

Lock then consumed residual despair, morphing into a colossal frog-like dragon with winged, spiked features. He initially overwhelmed the Cures but was defeated and purified by their Éclat Espoir attack, reverting to his fairy form, Kuroro. Purified Kuroro possessed cream fur with orange markings, dark green eyes, and burnt orange hair. Trauma over his actions made him withdrawn and guilt-ridden, avoiding others due to fear of their hatred. Through interactions with Miss Shamour and the Cures, he gradually accepted friendship and sought redemption.

Dyspear temporarily resurrected Lock's corrupted form using Noble Academy's despair to amplify his power. He overpowered the Cures until collective hope weakened him, leading to his final defeat. Kuroro later expressed sorrow for the coat's fate. He then voluntarily re-fused with the coat to shield the Cures from Dyspear, enabling their transformation.

Post-series, Kuroro returned to Hope Kingdom for royal fairy training under Miss Shamour, dedicated to self-improvement. His abilities evolved to focus on defensive support during battles, contrasting his earlier offensive role. Kuroro's path spanned transformation from a brainwashing victim to a figure seeking atonement.