Realm Souls Gaiden (レルムソウルズ外伝) is an action role-playing video game developed by Burst Arts for the Nintendo DS. It is the fourth installment in the series, and takes place near the end of the first game, continuing parallel to Realm Souls II: Dimensional Silence. The game was released worldwide in 2010. The story is told from the perspective of Kozor, following his daily life within the League of Fifteen and his relationship with League member Zeki; it also introduces Jeri, a rogue slime-like Anti-Soul who befriends the former two.
Realm Souls Gaiden features the combination of action role-playing game/hack and slash style of the previous games in the series. To accomplish this, the game makes minimal use of the system's touch screen functionality, and it is possible to play and complete the game without using it at all. The game includes single player and multiplayer modes of play. The single player mode is story-based, the player controls Kozor, who is accompanied by other League of Fifteen members on daily missions to video game worlds from the previous games. The two modes are linked in that character growth is shared between them.
Gaiden features a new gameplay mechanic known as the panel system which involves the customization of the character's levels, items, equipment, and abilities. For example, certain magic attacks cannot be used if their corresponding panels are unequipped, but can be used consecutively if several are attached. Some panels can occupy different slots, but can be combined with other panels to form new or stronger abilities.
The multiplayer gameplay is separate from the single player story and is mission-based, including cooperative and competitive player battles. Up to four players can play as one of the fifteen members of League of Fifteen, with each member having different weapons, skills, and stats. Rook, Mario, Luigi, Naoki, Sue, Winona, Baron, Mona, Len, Hyoma, and Geno are featured as secret characters in the multiplayer who are unlockable while playing through the main story.
The game is centered around Kozor, the Void of Rook (who transformed into an Anti-Soul to save Sue's life in the first Realm Souls) and the fifteenth member of the League of Fifteen. Each day he is sent on missions to other worlds, either alone or accompanied by his fellow members, to destroy Anti-Souls with his magic weapon, a replica of an ancient Gunsword, and release captive souls, with which the League can fulfill their goal of summoning Realm Souls and become humans again. But between his visions of Rook and the sudden twist of his objective, he begins to question the motives of the League of 15...
- Jeri – A slime-like creature that was found roaming in one of the worlds Kozor was sent to during his missions. After finding the creature, he decides to take it in, but also keep it out of the League's sight. Since then, Jeri has joined Kozor in a majority of his solo missions. Later it is revealed that Jeri is an Anti-Soul, but not a generic type of Anti-Soul; it is classified as a Top-level Anti-Soul based on its strength. Throughout its life, it encounters and interacts with Len and Hyoma in its break time several times and forms a bit of a bond between them.
- Paradox Tower (Realm Souls)
- Obsolete World (Realm Souls II)
- Crescent City (Realm Souls II)
- Fortune Isles (Realm Souls)
- Zebes (Metroid)
- World Tournament (Street Fighter)
- World of Weaponry (Soul Calibur)
- Kong Island (Donkey Kong Country)
- Hyrule (The Legend of Zelda)
- Mega City (Mega Man)
- South Island (Sonic the Hedgehog)
- Demon Village (Ghosts 'n Goblins)
- Nightopia (NiGHTS into Dreams...)