All are supposedly very powerful. Slayers are treated as walking Gods by FUG members. SIU noted that the latter blindly follow the former, regardless of whether their wishes are good or evil.
The Slayers of FUG are divided into three types, depending on their time of appearance:
Firstly, there are those like Luslec who emerged in the age of the 10 Great Families - they are known as the ancient gods.
Secondly, there are those who once again burned their names into the memories of the Tower's residents when FUG had become a power completely submerged in darkness - they are the demons of the 2nd generation.
Thirdly, there is the current generation of Slayers who are active in the present.
Slayers are treated as walking Gods by FUG members.
—From SIU's blog
FUG is an organization shrouded in shadow. They have no visible physical forms or headquarters, but the higher-ups are of the High Ranker level and most likely possess control over part if not all of some Floors: for example the Bayroad fighting pit and Karaka's hideout on the 97th Floor. As those are hidden places in the Tower, we can assume that the other places of gathering or control of the group are just as secretive.
FUG is often compared to the Wolhaiksong because of their opposition to the 10 Families. However, whereas Wolhaiksong is more of a "social club" centered around several members, FUG is a "religious organization" centered around a single faith: to kill Zahard and the heads of the 10 Great Families. Although the relationship between the two organizations is not that good, there are a few members who are associated with both groups.
The 10 Families forbid anyone within their Families to enter FUG and they consider it an important duty to eliminate their members, who are known to hide within the shadows of the Middle Area. Despite its dangerous status, it does not mean that they hold enough influence to challenge Zahard and the 10 Families. As long as the 10 Family Heads hold the contracts with the Tower’s Guardians, FUG’s influence cannot overcome the Family Heads regardless of how strong the organization gets. In particular, Zahard holds such a contract that prevents the inhabitants of the Tower from killing him. In order to overcome this matter, the organization waited a long time for someone free from the Guardian’s contracts: an Irregular (such as Jyu Viole Grace), unbound by the contract and thus suitable to fulfill their wish.
The structure of FUG is slowly appearing as the story goes. At the top are the Slayers who are considered living gods. The number one Slayer is Grace Mirchea Luslec, also called "God of the Devils". Then there are the Elders, who take decisions with the active Slayers on the great plans of the group, for example, appointing specific personnel for delicate missions; according to Evan Edrok, they are more mysterious than the Slayers. Then there are High Rankers who hold specific positions and power within the group (e.g. Ha Jinsung who is a martial arts teacher of the Slayer candidates). Slayers have command over executives (probably at least at the level of a Ranker; for example, Karaka commands over Poken) and even armies.xecutives can have dormant Regular agents planted on different Floors that wait, sometimes for thousands of years, until they are called to accomplish their missions (e.g. Reflejo). Finally the executives can recruit Regulars that are fervent believers in FUG or FUG's causes (e.g. Xia Xia). Though there seems to be a structure and a head to the organisation, that doesn't prevent dissension among the group. As for now, a splinter centred around a few Slayers and Elders has decided and put forth a plan in order to alienate the powers of Jyu Viole Grace and the Thorn.
Note: Some characters may be listed here if it's unclear whether they're related to FUG.
Beniamino Cassano: A product of the Living Ignition Weapon experiments. He was part of Team FUG during the Workshop Battle but was seemingly only doing so for his own reasons. His current connection with FUG isn't clear. He delivered Emile to Rachel's team and has been part of it ever since.
Traveller: A traveller lured by Emile to the Archimedes. He has been travelling with Cassano after the latter showed him the true nature of Emile.
Notes and Trivia
While the proper name of the organisation is unknown, some readers believe it to be "Forgiven Under Grace", titular to its founder, Grace Mirchea Luslec, or "For Unforgettable Grace". SIU confirmed that the G in FUG does stand for "Grace" (though perhaps not for Luslec).
Actually, besides Regulars, only people with permits can enter the Regulars' district. Hwa Ryun was only able to enter and bring Ha Jinsung along because she is a special Guide. Rankers and High Rankers are actually forbidden from associating with Regulars. Of course, FUG is a random bunch so they don't care about the rules.
The story of Tower of God actually revolves around the 10 Great Families and Zahard’s past. Why FUG is trying to kill Zahard, how Zahard became the King of the Tower, why Grace Mirchea Luslec wanted to make Baam a Slayer. All of these will be revealed one by one (as well as stuff about Zahard's Princesses). These will be explained over a long period of time in Season 3 and 4.
FUG is the only one getting worked up about everything. However, in the 10 Great Families, only the Family Heads are truly invincible, so FUG is an incredibly irritating organisation for members of a Family since it constantly commits terror acts directed against the 10 Great Families. And when they get up to the level of a Slayer, even the Family Heads start to get a bit sensitive.
According to Baam, it is rare for a Regular to be chosen as a Slayer Candidate.
Baam thinks he has already seen Karaka's silhouette somewhere. Yet, when they do meet, Baam does not know his name, and Karaka asserts that it is their first encounter.
FUG didn't capture Baam because of his talent, but rather due to his Irregular title.
FUG’s slayers are special and clearly differentiate themselves from normal High Rankers.