The Oathkeeper is a Keychain for Sora’s Keyblade which appears in Kingdom Hearts, Kingdom Hearts: Chain of Memories, Kingdom Hearts II, and Kingdom Hearts 358/2 Days. It also appears as half of Roxas‘s dual set of Keyblades alongside the Oblivion, and was transformed from his original Kingdom Key after he absorbed Xion. Fundamentally, the Oathkeeper is symbolic of Sora’s memories of Kairi.
The Oathkeeper is formed when Sora uses Kairi’s Lucky Charm as a Keychain. After rescuing her from Hollow Bastion in Kingdom Hearts, Kairi gives him the charm for luck in the Secret Waterway in Traverse Town. When Sora enters Castle Oblivion in Kingdom Hearts: Chain of Memories, Naminé rewrites Sora’s memories, replacing Kairi’s Lucky Charm with what appears to be a trinket from herself. After Sora defeats the Darkside at Destiny Islands and speaks with Naminé, she reveals the truth, and the illusion is dispelled. Although Sora continued to possess the Lucky Charm throughout Kingdom Hearts II, he is not able to use the Oathkeeper until he returns to Twilight Town and learns where Kairi had been taken. The Oathkeeper’s keychain token is Kairi’s Lucky Charm, a star sewn together from Thalassa Shells. Its shaft displays two hearts, and its handle bears two angel wings, contrasting the bat wings that comprise Oblivion’s handle. The teeth are fashioned in the shape of the Kanji for “light” (光), corresponding to the Oblivion’s kanji for “darkness”. When it is summoned by the user, it emerges in a bright, yellowish-white light
The Oathkeeper has a decent reach and is well-balanced in Kingdom Hearts; Oathkeeper is a good substitute for the Lionheart, in case the player hasn’t been able, or didn’t want, to defeat Cloud and Leon in the Hades Cup. In Kingdom Hearts II, its ability of Form Boost makes it much more useful for leveling Sora’s Drive Forms, rather than for regular battle.