Isaac

"Those without killing intent are worthless! Come at me bro with murderous intent!"

- Isaac, Volume 2 Annex

Isaac, also known as Black Sword Isaac, is the guild master of Black Sword Knights. He is a Guardian and a member of the Round Table Alliance.

Villain in Glasses
Isaac is seen near the end of the episode, having received an invitation from Marielle and Shiroe to attend the Round Table Council meeting. He comments that he is interested in the meeting solely because Shiroe had a hand in the operation.

West Wind Brigade side-story
Soujiro meets up with Crusty, Isaac, and Michitaka, since the three battle guild leaders were having their weapons repaired by Michitaka. Isaac greets Soujiro as "West Wind" and adds that it was unavoidable to encounter enemies. After the meeting, Soujiro accompanies Isaac to the latter's guild hall, where Isaac attempts to recruit Soujiro. When the boy refuses, Isaac tells him that anyone who didn't join the Black Sword Knights was its enemy, and that he would kill Soujiro the next time they met.

Predictably, it doesn't happen, as Soujiro approaches him soon after Woodstock fails to dissuade Isaac from using EXP Pots. After a brief scuffle, Soujiro persuades Isaac to assist him in taking out two PK guilds. When the mission is finished, Isaac leaves after reminding Soujiro that he would kill the West Wind Brigade the next time they met, and that he only aided this time for Akiba's sake.