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.

Appearance
Isaac is described as having "a head full of blood-red hair and a confident, contemptuous smile on his face." He is rather tall, almost equaling Krusty in height.

Personality
Isaac is a rowdy person who enjoys fighting, taking comments from his guild members like "You're really naggy, Commander," and "Chief is an idiot" in stride and treating them more like compliments. He tends to be easily irritated, but is not unwilling to listen to others. 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 realized that he only understood the surface of Shiroe's personality.

He's a nicer person than he is known to be, aiming for his guild to surpass level 90 to keep them from thinking about their current situation and being concerned over Akiba's fate. Soujiro says that Isaac is "just a little shy," although whether that was true or simply to annoy Isaac is unknown. Nyanta, after Isaac and some of his Summoners clean up Log Horizon's new guild building with their Undines' water, says that the Black Sword Knights are actually good people.

In battle, Isaac, like Krusty, acts in a way opposite of his normal behavior. Although he is normally hot-blooded and easily agitated, he is methodical and calculating in battle.

Round Table Conference arc
At the beginning of Resolve, Isaac is seen leading his guild as Shiroe talks about the capabilities of Guilds.

When Maryelle and Shiroe discuss the situation in Akiba, Maryelle 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.

Isaac is seen enjoying a Crescent Burger he bought from the Crescent Moon Alliance with Rezarick. Later, he is seen after receiving an invitation from Maryelle and Shiroe to attend the Round Table Council meeting. He comments to Rezarick that he is interested in the meeting solely because Shiroe had a hand in the operation.

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 guild masters have a shared history. 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.

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 Maryelle. 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.

When the Round Table Council meets to discuss Eastal's invitation, he once again looks down on the People of the Land as NPCs, only to be corrected by Rodrick. He is the one who suggests that a member of the Production guilds attend, and when the three Production Guilds leaders can't agree on who to send, Isaac and Soujiro suggest resolving it with Rock-Paper-Scissors.

Return of the Goblin King arc
At the start of the Goblin invasion as a result of the Return of the Goblin King event, Isaac is uninterested in assisting the People of the Land. Instead, he goes up to Susukino to help evacuate any Adventurers who wished to leave the city.

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.

When Shiroe realizes that there was a Person of the Land that was trying to sabotage the festival, he orders Isaac to have his men quell any fights that were turning violent. Later, during Raynesia's dinner party, he is among those in charge of protecting her.

West Wind Brigade side-story
Soujiro meets up with Krusty, 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 Maryelle, 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.

When Soujiro announces his plan to kill the PKers, Woodstock goes to confront Isaac again. Irritated that Woodstock had come back, Isaac reminds Woodstock that the latter always demanded things from others when he couldn't do it himself. In the process, Isaac reveals his motivation for having the guild aim to surpass level 90 - he saw it as the best course of action for his guild members, to divert them from the boredom of their current world. Once Woodstock mentions the EXP Pots, he initially defends the usage of in-game items before learning that Hamelin was abusing the younger members whom the EXP Pots were coming from. Although fazed for a second, he resumes defending himself, but Woodstock questions whether Isaac was really okay with Hamelin's actions. At that moment, Soujiro arrives to point out that Isaac really did feel guilty about what he was doing, that he was nice guy who was "just a little shy," and that he cared about Akiba despite putting the guild first in his priorities. Soujiro's constant pestering causes Isaac to snap and he tries to attack Soujiro, who simply dodges it. He finally relents and decides to make their PK-hunting quick, only to yell at Woodstock again because the latter was not coming along.

When the PKers are driven into a dead-end by the West Wind Brigade, Soujiro and Isaac show up to kill them, with Isaac calling them pathetic for resorting to Player Killing. When some of them decide to attack Isaac first, he mocks them for their decision before revealing his sword's special ability, which forced his opponents to focus on him. He then easily slaughters them as Soujiro takes out the rest. Afterwards, he tells Soujiro that he wouldn't help him anymore, but would hunt any PKers that wandered into his guild's hunting grounds, and points at Magus while telling Souji that he would continue buying EXP Pots. He then leaves, allowing Soujiro to confront Magus alone.