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War Demon Shusui is a Non-Player Character (NPC) in the game. He is the first of the eight Demon Lords that the Summoner meets in Ishgria, as well as the ruler of Celgrad.
Shusui makes his first appearance in the story in the Lava Bridges within Celgrad, and ends up fighting the Summoner.
In the storyline
As Shura, he spent harsh days training under Raaga's guidance, along with his senior whom he idolized, Hisui. One day, Hisui challenged him into a duel. Shura won the duel by killing Hisui. Although Hisui didn't blame Shura for the fatal wound he received, it was said that the result hit Shura really hard, that he became obsessed with the title "The Strongest" afterwards. Shura then inherited a part of Hisui's name, and continued his training.
After Shura became known as the War Demon Shusui, he resided in Celgrad, testing anyone who comes to Ishgria to see if they are worthy of this land.
Several years before the Summoner stepped into Ishgria, a group that consisted of seven Summoners arrived. Shusui halted them and quickly engaged in a fierce battle with the leader of the group. The fight between them lasted several days, until the leader of the group finally died. Even so, the memory of the battle remains in Shusui's mind, as he told the Summoner and Paris later on.
When the Summoner arrives in Celgrad, He tests them and Paris, to see if they are worthy of entering Ishgria. After the Summoner defeats him, he remarks that he will be waiting at the Soaring Tree Tower, where he will be waiting for another fight, then runs off.
When the Summoner and Paris arrives at the Soaring Tree Tower, he appears to them and tells them that he will be waiting at the top of the tower in order to test them some more before their fight. After they defeat him in combat, he reveals that he doesn't want to invade Grand Gaia, since he only desires to fight the strongest opponent. He then reveals the existence of the other seven Demon Lords, mentioning that not all of them want to invade Grand Gaia, and heads them in the direction of Lem, before running off again.
He makes another appearance in Lem as a means to test the player before they face Mora in combat, and when defeated, leaves the region in order to test Owen. His next appearance is in the region Beiorg to test if the player's strength matches that of Owen, then runs off again after he is defeated. He makes yet another appearance in Wulgee to test the player yet again, saying that they have a long way to go before they match his true power, then runs away after yet another defeat.
While not appearing in the next region, Rakshult, Lugina mentions that he encountered Shusui in the Weather Tower, but didn't manage to defeat him before running off again. He makes another appearance in Zamburg to face Mora in combat to interfere with her plans. The battle was left ambiguous, but both of them still survived.
Shusui makes his last appearance in Gate of Abdication, where the Summoner and the party found him in a bad state. Despite his wounds, he still insists to fight with Summoner. After his loss to the player, Mora arrives and absorbs all of Shusui's power, effectively killing Shusui.
Shusui briefly appears in Raid Battle X6 - Maw of Madness as a raid boss in Mission 5 - The Price of Oaths.
As one of the Demon Lords who sealed Karna Masta, Shusui can use a massive AoE attack called "Empty Seal".
The Demon Champion Raaga was Shusui's master, from whom Shusui learned the art of fighting and the desire to fight strong opponents. Before his death, Shusui indirectly mentioned Raaga's name (calling him "Master").
Hisui and Shura trained under the same master. Shura idolized Hisui like a brother. However, Shura then killed Hisui in a training session. Although Shura knows the reason behind the training, the regret he felt led him to be obsessed with the title "The Strongest". After Hisui's death, Shura took a part of Hisui's name, and he was known as "Shusui" since then.