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.

Personality
Isaac is a rowdy person who enjoys fighting. He invited Shiroe to join his guild a few months after Debauchery Tea Party was disbanded but got turned down. After forming the Round Table Alliance, he realised that he only understood the surface of Shiroe's personality.

Resolve
At the beginning of this episode, Isaac is seen leading his guild as Shiroe talks about the capabilities of Guilds.

When Maryele and Shiroe discuss the situation in Akiba, Maryele tells him that the Black Sword Knights and the Silver Swords are aiming for level 91. She seems to hold a rather low opinion of Isaac, saying that "the Black Sword Knights always have a thing for being the best," and characterizes him as a person who is "proud, or maybe he just enjoys excluding others" due to his strict requirements for recruiting new members. Shiroe comments that Isaac is trying to compete with the D.D.D, which simply absorbed smaller guilds, by using "quality over quantity." However, this presented another problem—they would have to fight monsters level 85 or above, which is difficult after the Apocalypse.

This brings the two to the main issue at hand: the guild's usage of EXP Holy Water, acquired from Hamelin through the latter guild's abuse and usage of low-level players that they tricked into joining. This leads to Shiroe's determination to both better the city and to destroy Hamelin.

Crescent Moon
Isaac is seen enjoying a Crescent Burger he bought from the Crescent Moon 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.

Round Table Conference
Isaac and the other guild masters meet at the Guild Building to attend the Round Table Conference. When Shiroe, as the organizer of the meeting, begins to introduce himself, Isaac identifies him as "Tea Party Shiroe" and tells him to keep his introduction short -- thus revealing that the two guildmasters have a shared history.

However, after William's departure, Isaac argues with Shiroe over what to do about the current situation in Akiba. After Shiroe reveals that the Guild Building is under his control and states that he wishes to create laws for governing all the inhabitants of Akiba, Isaac demands to know what else Shiroe has planned for his guild Log Horizon. In response, Shiroe simply adjusts his glasses and smiles enigmatically.

Grab it in Your Hand
Isaac angrily demands for Shiroe to reveal his plan. He calms down after Michitaka and the other two Production guild masters reveal the development of a steam engine, wondering why it was so important. When the subject of People of the Land is brought up, Isaac voices many players' opinion that they were merely NPCs that had no value, but was proven wrong by Maryele. He later agrees to the foundation of the Round Table Alliance and stands with the other guild masters when they reveal it to the populace.

When Log Horizon moves into their new guild building, Isaac pays a visit with some of his summoners. Under the pretense of training them with the large space of the building, they summon Undines and flood the place, cleaning it in the process. They promptly leave afterwards as Nyanta comments that he isn't such a bad person.

Libra Festival Arc
During the Libra Festival, the Black Sword Knights and D.D.D are in charge of border patrol, checking on People of the Land's carriages and contents, and making sure that everything runs smoothly. Isaac sits in the middle of the Black Sword Knights' tent, giving orders to his men and taking their reports. While he mentally congratulates himself for being a good leader, Krusty comes to check up on him.

When Krusty sits down next to him, Isaac ponders about Shiroe, revealing his past with the Enchanter. He had observed the Debauchery Tea Party and its disbandment. Shiroe, as one of the few players from the Tea Party that continued playing regularly, joined on some raids as a mercenary. Intrigued by Shiroe's tactical prowess and playing style, Isaac attempted to recruit him to no avail, and gave up. After Krusty gets him to understand Shiroe more, Isaac decides to take a walk around the city since just sitting there was getting boring.

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 is leaving with Isaac when two members of the Black Sword Knights approach them, telling Isaac that they were there to escort him. After waving off the two members, Isaac suggests having his guild merge with Soujiro's due to the uncertainty of the world around them. Although Soujiro disagrees with Isaac's view of the world, the two members cut in to try and persuade Soujiro otherwise—although not in the way that Isaac would like, as their primary concern was the lack of women in the guild. After Soujiro reiterates his decision not to take up Isaac's offer, Isaac leaves after reminding Soujiro that this made them enemies.

After Soujiro's brief run-in with Magus, Magus meets up with Isaac in a cave to deliver EXP Pots.

He is mentioned in the meeting between Maryele, Akaneya, Woodstock W, and Soujiro after Hamelin and Magus are brought up. It turns out that Woodstock had already gone to Isaac to try and dissuade him from using the EXP Pots. As Woodstock had expected, the discussion turned sour and Isaac refused to budge on his position. Woodstock bemoans that the guild hadn't changed at all since he had left, which was before the Apocalypse.

Later, Soujiro approaches him soon after Woodstock again 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.