The Assassin's Guild (組織, Soshiki in Japanese, meaning "The Organization"), also known as the Assassin's Syndicate, or just "The Guild", is a shady group composed of assassins. It's been around for a hundred or more years, and works in secret and has huge networks across different countries. Many of its members have battle prowess similiar to that of Gears, and it is suggested that the practice of "Kinju" (Forbidden Magic) has taken place at The Guild.
Though it wasn't this way at first, the Guild has its hands in politics; it is certainly able to influence the politics of A Country (formerly the United States) and evidence points that the Guild may in fact be based in A Country. They are also known to make alliances with corrupted officials. Many presidents of A Country are just "puppets" of the Guild, specifically the current one, Erica Bartholomew, though she has formed an alliance with Zepp to decrease the Guild's power of politics.
The Assassin's Guild was founded by Slayer to, as he puts it "punish those who consider themselves above the law". However, it was at this point that Slayer grew tired of the way of life on Earth and "retired" to his castle, an area that is yet to be seen. Many years passed, and eventually Zato-1 became the new leader of the Guild, presumably thanks to the Forbidden Beast implanted in him by the Post-War Administration Bureau, which greatly increased his fighting capabilities and heightened his senses far more than his eyes were originally capable of.
He had also taken in Venom and later Millia Rage, both orphans, under his wing. Venom was nearly killed by the Guild, as his natural aptitude for killing was hindered by his hatred of murder. However, Zato appeared and saved his life, and Venom has been devoted to Zato ever since. Though Venom was most likely a child when he was brought in and saved by Zato, it's unclear what age Millia was brought into the Guild, nor if the Forbidden Beast, Angra was placed in her at the same time as Zato, which would heighten the chances she was a young adult when she was brought in. Millia didn't want to be an assassin, but her relationship with Zato caused him to manipulate her to stay.
However, during a mission, Millia grew sick of the way Zato used her, and abandoned Zato while they were surrounded by enemies, though he managed to escape. Millia had then fled the Assassin's Guild, and, wishing to leave that part of her life behind, cooperated with the International Police Force to catch Zato-1, if in exchange Millia would be acquitted of all charges of assassination and removed from all formal and informal bounty lists if they could catch the ringleader of the Guild. Indeed, they did catch Zato-1 and put him in the dimensional prison. Zato was imprisoned for quite some time, eventually causing Zato to obsessively curse Millia's name, while still harboring feelings for her as a woman.
One day, however, Testament who had ties with the PWAB at the time, utilized their resources to free Zato-1 and have him enter the 2nd Sacred Order of Holy Knights tournament. The whole purpose for this was of course for him to shed as much blood as possible, being an assassin. Millia and Zato assumedly confronted each other at the tournament, though it is unclear what exactly happened between the two, but it can be assumed at this time, Zato had already went mad from the powers of the Forbidden Beast, and both failed to kill each other.
Some time later, back at The Guild, Millia would find herself working alongside Venom (begrudgingly, albeit Venom is treated like a junior to a senior by her) for the same purpose of tracking down what remained of Zato... either as a human or a Forbidden Beast. At this point (Guilty Gear X), he was neither alive nor fully dead. His body would be completely dead by Guilty Gear XX however, as Eddie depleted Zato's life. At some point after Eddie's departure from the Guild, Slayer had appeared, as he wanted to disband the Guild, since he believes it has lost its original purpose and has turned into something that not even he wanted. What was originally an organization that desires justice (and in turn provided a consistent blood supply), was now a mercenary-like group that accepts any dirty job.
Venom, however, wanted to take over as the leader of the Guild in Zato-1's memory, so decided to appoint himself as the new de facto leader of the Guild, without going to Slayer first. Both eventually confronted each other, and Venom identifies himself as the leader of the Guild, prompting Slayer to say the Guild "no longer exists". Venom replies by saying that the Assassin's Guild is now a monument of Zato's power, and that he will kill Slayer if it means protecting it. Slayer says he is not weak enough to lose to a human, and the two fight. Afterwards, Slayer says that Venom's skills do back up his words, but he'll need more to defeat Slayer. In one GGXX ending, Venom seals Slayer in a chained coffin.
The other two endings have Venom never confronting Slayer. In ending 2, Venom finds Eddie, and kills him, and decides to take a "rest" (die) next to Zato. In ending 3, Venom meets up with Millia and both fight, with Venom killing her. As GGXX was a type of gaiden installment, none of these endings are considered canon. Accent Core Plus, as in this game, Slayer is not sealed away, Eddie is still alive, and Millia is still alive as well. Because of this, it would seem that Millia's first ending in Guilty Gear XX is canonical with the Guild's future, with Millia attempting to confront Eddie, but with Slayer blocking her way. After a fight, Millia is not strong enough to get past Slayer, who "reveals" to her her hair's power is of the same origin of Eddie, though Millia "knows this better than anybody".
In Millia's AC+ story, one ending has her eventually confronting Eddie, and killing him. Eddie asks if Millia will keep him (Zato) alive in her memories, and fades out of existence. In Eddie's other ending, after a fight with Venom, just when Zato's body should have decayed to the point of no use left, Eddie doesn't die. Eddie then realizes his morality is different then a human being's, and strikes Venom down before leaving Zato's corpse to go live on as an independent creature. It's most likely Millia's first ending, in which she kills Eddie, is the canonical one. Venom's story in Guilty Gear XX Λ Core Plus reveals that shortly after bringing the Guild back from the brink of destruction Venom could find no joy in his work, and (in one his endings) disbands the Guild himself and begins traveling the world as a pool player.
The status of the Guild changes significantly during the events of Guilty Gear Xrd -SIGN-. Venom is still the leader and is working on making the organization a more legitimate enterprise. He receives a mysterious contract from the Conclave, which has a very tempting reward: the revival of Zato-1. Skeptical of the offer, he contacts Millia and has her investigate while he carries out their orders.
How exactly the Guild works is largely unknown. It appears that members of the Guild have to wear some kind of black, hooded uniform that obscures much of the assassin's face. Though it's is unknown how exactly recruitment or teachings go in the Guild, it is known that the Assassin's Guild sponsors orphanages, and pick of the children with the most promise or fighting prowess. there is no mentioned age requirement to begin training within the Organization, though more often Children are selected due to the fact they can be trained early on. You technically don’t have to be an Orphan to join.
The reason they selected orphans is because during the Crusades orphanages were a good resource for selectable candidates. Usually children with unusual talents or perhaps a violent or delinquent disposition were selected. Alternatively, they also recruit powerful warriors, for example, Slayer attempted at one point to recruit Chipp and Baiken. If one displays no talent in the fields of assassination, they will indeed be killed, as seen by the Guild nearly killing Venom. This presumably goes for people of all ages. The only known rank within the Assassin's Guild is the leader.
The Guild employs several tactical methodologies when sending their subordinates (or even their leaders) out on a mission. Usually this involves a long span of time, gathering intel (information) about the hit site, security levels, potential danger zones, escape routes, and a plethora of other bits of useful information they will need when starting their missions, which range from Assassination, to Kidnapping, to Extortion, to military or political ruses (framing someone for example), or simply tailing and gaining confidential information on a subject matter. The second phase of a mission usually involves deployment types. Depending on the mission’s priority, they can utilize one, two, or as many as a small army of Assassins as either a front-line force or a deterrent (a distraction).
Key to most missions is the Striker…that is to say, the operative who will act on the hit (the kill). While there are situations that are simple enough for a solo-strike operation, most missions are handled with the consideration that things may go wrong and that there must be contingency plans to fall back on…so where one Assassin may be given the mission, two more may take the operative’s place should he/she fail…these additional operatives are called “Support” operatives.
Support usually do most of the intel work and relay it to the Striker if necessary, and in some cases provide backup to the Striker either directly or as a deterrent. However, they also serve a hidden purpose…should the Striker for any reason be compromised (such as being captured by the enemy) or disobey orders, or depending on the situation, compromise the entire mission, the Support operatives are sent in to silence the Striker and remove all incriminating evidence tied with the Guild.
Support isn’t limited to two people, but can be as large as a small army if backup situations call for it.
It should be noted that in Tsuyoshi and Millia Rage’s case, small armies of assassination groups WERE deployed in order to eliminate any threats to the Guild, however in both cases were unsuccessful in eliminating their targets due to sheer strength differential (that is to say Millia and Tsuyoshi were still wanted fugitives from the Guild at some point). As seen by Volf, if a trained assassin fails missions too many times, they are executed. Also, if an assassin betrays the Guild in any way, another high ranking assassin will be sent to kill them, as seen with Tsuyoshi.
- Slayer - Founder of the Guild, though left shortly after its creation, growing tired of life on Earth. Later returned to disband the Guild, as Slayer felt the Guild had lost its purpose and turned into something not even he had intended. However, after fighting with Venom, he decided to appoint him as the new leader of the Guild, and then left forever.
- Zato-1 - Once just an ordinary assassin within the Guild, Zato became the leader of the Guild presumably by the use of a being known as a "Forbidden Beast" that was implanted within him by the Post-War Administration Bureau.
- Millia Rage - Millia was an orphaned child, taken in by the Assassin's Guild and trained how to fight like an assassin. However, she had no interest in a life like this, and tried to kill herself several times, each ending in failure. After growing up into an adult, and having a confusing relationship with Zato-1, she eventually abandoned Zato in the middle of a mission while surrounded by enemies and fled the Guild, not wanting to be an assassin anymore. She later had all charges and bounties on her head cleared in exchange for for helping the Internation Police Force find Zato-1.
- Venom - Venom was yet another orphaned child brought into the Assassin's Guild. However, when the child lacked the will to kill others, he was to be executed. However, Zato took the child under his wing, saving his life. Ever since, Venom has been devoted to Zato. Venom later became the new leader of the Guild after fighting with Slayer, who wanted to disband it. However, in one of Venom's endings, he himself disbands the Guild when he finds no joy in his work, though it is unknown if this is canon or not.
- Volf - An assassin said to be a subordinate directly under Venom. He was the man responsible for killing Tsuyoshi, Chipp Zanuff's master. After failing to kill Chipp several times later, he was personally assassinated by Venom.
- Tsuyoshi - Worked as an assassin for a short time at the Guild, but was secretly a member of the International Police Force, who were trying to investigate the Guild. His cover was eventually blown, and Volf was sent to assassinate him. However, Tsuyoshi evaded him and ran into Chipp. Both moved up into the mountains, until one day, Volf found where Tsuyoshi was living and managed to kill him while Chipp was off getting something.