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Sumeragi Youta (住良木 陽太, Sumeragi Youta?), known as Sumeragi (スメラギ, Sumeragi?) in «ALfheim Online» (ALO), is a game-original character featured in the game adaptation Sword Art Online: Lost Song. Sumeragi's ALfheim Online avatar is an Undine. He wields a katana named Ame no Murakumo, which he can brandish with one arm due to his strength. He has created his own Original Sword Skill, «One-Handed Tyr».
Sumeragi is an Undine swordsman, who wields a katana, and a member of the «Shamrock» guild. While Seven is the official leader of this guild, Sumeragi holds a de facto leadership role within it. He has apparently bested the Demonic Sword Gram.
Sumeragi is Dr. Nanairo Arshavin's (Seven's) assistant in real life. Later on, it was revealed that Sumeragi was secretly working with Kikuoka Seijirou in order to log Seven's activities, and he knew that Rain had actually obtained information about Shamrock's activities from Kikuoka Seijirou.
Sumeragi's first major interaction with Kirito was when he discovered that Rain, an ex-member of Shamrock, was using information she had stolen from the guild for her own use. He was somewhat surprised to meet Kirito face to face, as he remembered word spreading about a group of players, led by him, clearing the dungeons at an alarming rate. Rather than being upset that Kirito and his group had gotten the lead on them in a dungeon, Sumeragi respected them for it. Kirito's words piqued his interest, as the spriggan referred to their competition as a rivalry. Sumeragi agreed to duel Kirito, which they did in the following dungeon. However, the duel ended abruptly when Sumeragi realized that Rain was helping Kirito's party, and he exposed her deed to them. Seeing that there was no further point in continuing the conversation, Sumeragi insisted on Kirito's group leaving the dungeon, which they did.
Sumeragi once again arrived too late to the battle against the boss of Flosshilde, which was defeated by Kirito's party, opening the path to the Dark World, the next island. When Kirito inquires about Seven's well being, Sumeragi felt insulted, and responded by declaring that Kirito had no need to know that. This angered Kirito, who claimed that she was forced to uphold a lot of expectation from the adults around her. With the argument ending on ill terms, Sumeragi left.
Later, Kirito met Sumeragi, along with Seven, again in the final dungeon of Niflheim. Sumeragi applauded Kirito's group for their progress, and also apologized for the reception other Shamrock members had given them earlier. Sumeragi stood alongside Seven, as she explained her goal of perfecting the Cloud Brain to Kirito. When Kirito refused Seven's invitation to join Shamrock, Sumeragi declared that they would settle the matter the next time they met.
As Kirito's group reached the boss floor on the second floor of the final dungeon, Sumeragi waited alone to face Kirito in their final duel. Kirito agreed to the duel between the two of them outside Niflheim's dungeon, and chose the Total Loss duel variant. During the battle, Sumeragi showcased his Original Sword Skill, but was defeated when Kirito used his Skill Connect technique with his twin blades to unleash a blistering combo. Acknowledging his defeat, Sumeragi allowed Kirito to proceed, but warned him that he could not defeat Seven. As Kirito's party advanced, Sumeragi sent full custody of his OSS to Seven using his Shamrock badge. But despite giving Seven his Original Sword Skill, she was still defeated, and the experiment failed.
With Seven defeated, Kirito met with Sumeragi several days later, and confirmed his suspicions that he was secretly working with Kikuoka Seijirou unbeknownst to Seven. Despite enjoying his duel with Kirito, Sumeragi claimed that as long as Seven was truly happy, he did not need to stay in ALfheim Online, and thus chose to quit the game, but not before Kirito told him that he was welcome back, as Kirito wished to duel him again.
Sumeragi later joined the group for the event battle with Lost Song.
- Sumeragi can be later unlocked as a playable character in Lost Song if Kirito goes to the first island and speaks with him alone.
- In Millenium Twilight, Sumeragi gives Takumu his OSS after a duel.