Traveler

The Travelers are an artificial, non-corporeal race of unknown origin that arrived in Theldesia during the Catastrophe (which they call the Eclipse), just like the Adventurers. This race is split into two factions: Fool and Genius.

It is important to note that "Fool" and "Genius" are the way their names are supposed to be pronounced and were written as ruby characters. Their actual kanji mean "Inspector" and "Gatherer," respectively. In the anime the chosen terms are "Observer" and "Harvester".

Overview
The members of this race did not have physical bodies to begin with, so they had to take bodies that already existed in Theldesia (either accounts that were inactive at the time of the Catastrophe or the bodies of monsters).

The Genius intend on focusing their efforts on destruction and chaos while gathering Empathium (as their name suggests), while the Fools seem more interested in observing the other players as well as the physical sensations and experiences that Elder Tale has brought to them (likewise, as their name suggests).

They have a better understanding of the Catastrophe than the Adventures, however Roe2 admits they don't have a solution for it.

Prior to the Catastrophe
The Travelers are an artificial life form created in order to find Empathium. The world that their creators came from had run out of Empathium before they fully realized that Empathium was necessary to create more Empathium; as a result, the Travelers were created to search out Empathium from different universes. They created something known as the "Backdoor," a mathematical phenomenon that would be used to find other universes with Empathium. Using it, they were involved with the Catastrophe, knowing full well what would happen.

The universes are usually tied to several common rules. First, only one intelligent life form exists per universe. Second, the Law of Providence Horizon states that no two intelligent life forms can coexist in the same universe. Third, the Law of Contradiction dictated that before a contradiction could occur, one of the life forms would cease to exist. As a result, normally the Travelers' search would end in failure. However, the world of Theldesia was an exception as a result of the Catastrophe, which caused three forms of intelligent life to exist in the same universe. According to the Law of Providence Horizon, two of these three life forms should not exist here.

The Genius immediately began harvesting Empathium when they arrived on Theldesia as a result of their orders, while most of the Fools are observing.

Route 43 arc
Touya's Party, while traveling in the Sakawa Region, encounters a group of People of the Land facing a horde of Ogres when they hear a voice through the trees warning them of more higher level ogres being on the way. As the group is fighting the monsters, Roe2 appears out of the forest to join them in battle and summons a Princess Wraith to aid them in battle. She also formally introduces herself to the group and reveals that she is a vampire who is also travelling to the south in order to change her subclass, complaining that the person who chose it for her gave her a rather cumbersome one

She watches the invasion of Wyverns in the city of Saphir and the Odysseia Knights' repeated suicide tactics, which she believes to be part of the Genius plans, since it creates something called "Empathium." Minori asks who Roe2 really is. She answers that she is a Traveler and gives a long explanation beyond anyone's comprehension. In the end, she joins the battle against the Wyverns.

After the battle she hands Minori a letter addressed to Shiroe.

Kanami, Go East arc
In China, Kanami's Party hears reports that the Lelang Wolf Cavalry was wiped out by undying monsters. Upon hearing this, Kanami decides to go face down these monsters, with Elias Hackblade and Coppelia following her. Although Leonardo initially hesitates, he agrees to go after talking with KR. The party is successful in killing the two Genius monsters they face, although KR is killed and sent back to Yamato in the process.

The Genius also have a secondary objective: to destroy any means for the Adventurers to reach Mare Tranqullitatis and communicate with the Fools, so anything they find that could make this happen is to be immediately destroyed, such as the abandoned space station in Tone's Grave in the Chinese server. Because the Lelang Wolf Cavalry went to intercept them, the Genius were also able to harvest Empathiom off of the Adventurers.

Later, it is revealed that KR joined Plant Hwyaden in order to gather more information about these monsters. From his research, the Genius were moving around the server and walking around, unseen and unknown. One is in Nakasu, while two are in Akihabara.

Homesteading the Noosphere arc
Shiroe reads Roe2's letter, which was written while Touya's Party was in Sapphir. The letter explained that she is a Traveler, a race of mental bodies that were brought to the world of Theldesia because of the Catastrophe. The Travelers called the Catastrophe by a different name: The Eclipse (its kanji means "pact"). She continued to explain that her race has two factions: the Fools and the Geniuses. While the Travelers had an explanation for the cause of the Catastrophe, the explanation was not 100% certain, and they had no solutions as to how to return to their own world.

Other Media
In the Log Horizon TRPG Replay, the members of Nyanko-tei face and kill the Genius Monster called Sislau.

Fools Faction

 * Roe2
 * Tamasaburo (temporarily controlled by the Fool using Roe2's body)
 * Nantur (both a Fool and a Genius)

Genius Faction

 * Baglis
 * Camaysar
 * Heiglot
 * Mascarvin
 * Mizkun
 * Papus
 * Rasphuia
 * Sachluph
 * Sislau
 * Tacritau
 * Zahun