The vast and ever mysterious world of Tower of God includes several groups and organizations which are rarely seen but are powerful enough to be known and feared throughout the Tower. One such organization is FUG. FUG is known to be the most dangerous and secretive criminal syndicate in the Tower. Their influence in the Tower is powerful enough that Jahad and the 10 Great Families want their members eliminated.
FUG's main purpose is to kill Jahad and the heads of the 10 Great Families. This is not generally known among the people of the Tower, and though they operate from the shadows, their existence is well-known. Several key characters, including Bam and Rachel, are either affiliated with or full-fledged members of the organization.
After Rachel betrayed Bam by attempting to push him to his death, he was forced to join FUG against his will. He was told that his friends would be killed if he tried to leave. And so, he was obliged to do whatever FUG wanted in order to protect the people he cared about. He decided to take full advantage of his situation by using it to become stronger. He was trained by a powerful high ranker named Jinsung Ha. The two eventually became very fond of each other as they developed a master-student bond.
The structure of the organization is still not completely clear at this point in the series but certain details can be deduced based on what has been shown. Slayers are at the top of the pyramid and are considered living gods among the members of the syndicate. There are the Elders who make major decisions and develop plans, and there are high rankers like Jinsung Ha who hold specific positions, such as being the martial arts teacher for Slayer candidates.
There are meant to be 11 Slayers, most of which target a specific leader of the 10 Great Families and Jahad. However, currently, there are only seven with the other four seats being vacant. Bam is a candidate for one of these seats. It's worth noting that of the seven Slayers, only four are actually active. The top Slayer actually has a history with Jahad and the heads of the 10 Great Families since he helped them climb the Tower. He served under Bam's father who happened to be one of Jahad's companions.
FUG is by no means a group of people who work together because they like each other. They are a religious organization connected by one thing: their desire to eliminate Jahad. Many of FUG's notable members and affiliates are rivals or enemies and have tried to kill each other. For example, Bam has been both enemy and ally of the slayers Karaka and White. He is currently on good terms with Karaka through their shared bonds with their master Jinsung Ha. On the other hand, he and White are trying to kill each other.
The question of whether FUG is friend or foe has no simple answer. It truly depends on perspective and the individual characters since there's discord even among the members. The series' main protagonist is a member, and despite being forced to join initially, eventually rejoined of his own volition after previously leaving the organization. He is allied with several members, while others would rather see him dead. The one thing about FUG that's certain is that they want to overthrow Jahad and his cronies for the betterment of the Tower. In that regard, they could generally be considered friends of anyone who aren't those figures.