AU Information: changes from P3 canon.
Other info, in a more condensed form (or what we've worked out so far) can be found in fracta_anima's journal.
Persona info and basic information about Minato himself are in other entries.
Strega's History
Minato is the leader of Strega, instead of Takaya; in fact, they're mortal enemies at this point, though they used to be tentative friends. A long time ago, during the Moonlight Bridge accident, Minato was picked up by Shuji Ikutsuki (though he is not aware at this time it was him) and subsequently left in the hands of the Kirijo Corporation. Obviously noticing the boy has the potential to summon a Persona and knowing Aigis had sealed Death within him, he was likely hoping Minato would produce something for them to salvage the failed attempt to summon Nyx.
Unfortunately for Ikutsuki (and even more unfortunate for Minato), the boy was in shock, and as a result, was unable to summon anything at all on his own. You see, at the time, Minato had a younger sister to him by a year, almost to the day--Minami. She was injured very badly during that accident, and he was told by Ikutsuki that none of his family had survived. The truth was, she had lived, but was in critical condition. Ikutsuki wanted to keep her because she held one of the two halves of Death inside her, but he had to go to Minato first, since obviously he was in much better shape, and Minami had to recover.
Because he was told she had died, Minato went into a deep depression and a state of disbelief, which meant he couldn't manifest his Persona properly. Ikutsuki, disappointed and not wanting the boy's talent to be 'wasted,' handed him over to the Kirijo Corporation branch that had been doing experiments on kids in order to manifest Personas. Children were untainted by ideals and seemed to work best.
The experiments were cruel and inhumane; many of the children turned into Shadows. Minato, who had a mental block from the trauma of the accident, was then forced to summon a Persona he assumed was not his own. ... It really wasn't. That Persona would be Thanatos, which they forcibly pulled out of him, and it embodied the aspect of Death within him. ... But this came at a steep price.
There was a lot of experimentation involving drugs and electricity, among other things. The kids were put through so much in order to make them feel near death and summon a Persona that was not, essentially, theirs. Although Minato should have been able to, he was no different from the others at that point. ... But he couldn't stand to see their suffering. So he dried his eyes, and began plotting his escape from the labs. Eventually, he succeeded in escaping, along with the three other remaining kids from the group: Takaya, Jin, and Chidori, all of whom could summon Personas, but at a great cost: they had to take Suppressant medication so that their Personas would not kill them.
Minato's problem was Thanatos and how unwieldy it was. Though it was strong, it seemed to have a personality all its own without the Suppressants and at times, he found if he let his emotions get the best of him, it would manifest on its own.
They spent most of their time on the run, learning how to better control their Personas every Dark Hour by fighting Shadows. Eventually, Minato and Jin would begin running a site called Revenge Request, where anyone could request illegal activities (drugs and smuggling) or assassinations. However, Minato dictated the rules by which they would take these requests:
1. The requester must pay a fee to even put in a hit request.
2. They will not kill unnecessarily, and the target will go quickly and painlessly. No exceptions.
3. They are to be paid in full, no checks, only wiring money to their accounts. (This is where Jin takes over.)
4. They will not kill members of law enforcement unless they are corrupt. (It could end in their capture.)
5. They will research the hit thoroughly beforehand to see if they actually deserve to die.
6. Hits will only be carried out during the Dark Hour, and on their schedule..
7. They will not take more than a few hit requests in the same country before moving on.
Following these rules, Minato earned the site (and subsequently Strega themselves) an almost underground, cult-following as "angels of justice" according to others. The site itself became an Urban Legend. No one could track down the IPs or where it even came from, thanks to Jin's genius computer skills. And because they would kill during the Dark Hour, it was easy to never leave any trace of evidence; the target or targets would go quietly in their sleep. Usually, the people they killed DESERVED to die and were all corrupt or generally terrible people. This was Minato's way of being morally "good," in a sense.
Eventually, Takaya disobeyed these rules. A hit request was put out that Minato did not give an okay on--someone pissed off at their boss and drunk had put one out, and Minato could tell. He told them no after a little research, where they found out that while the boss in question was an asshole, it was to provide for his family at home, to which he was very kind to. ... Minato had a soft spot for family, obviously, having lost his, and he wasn't about to kill this person.
Takaya, seeing money to be gained, went ahead without their permission during the Dark Hour and slaughtered the man and his entire family in a slow and painful manner. Minato and Chidori, completely horrified at the way he had behaved, held a small session in which they decided it was finally time to kick Takaya out. He was a bad influence on Jin, they both believed, and thought this for the better. So they left him somewhere with a little bit of money, and disappeared from his radar as best they could, hoping to put it behind them.
Minato and the others also had a habit of stealing from stores and such during the Dark Hour. They don't see anything wrong with with it simply because they believe it's there to be used.
Around a year later, they'd find out that a hit had been put out on Mitsuru Kirijo, and go to investigate. Eventually, this leads to their discovery of the Specialized Extra-Curricular Execution Squad, or SEES for short. Once they'd learned SEES intended to end the Dark Hour, Minato abandoned the idea of trying to kill Mitsuru Kirijo, as much as he would have liked to, and began actively trying to stop them, once they had enough information.
As a result, SEES and Strega would clash nearly every Full Moon after the third one. Sometime later, Minato would come across Ken Amada, whom Shuji Ikutsuki had been trying to manipulate, and convince him to join their cause, as he wanted revenge against Shinjiro Aragaki. Of course, Minato's real motive here was to make Ken consider what he would do with his life AFTER that, and if it was worth it--he wanted to make sure the boy, at least, had a family who cared for him. Chidori, he, and Jin were definitely that. Thus, Ken became like the little brother.
Eventually, Chidori would run across Koromaru fighting off a Shadow, critically injured. Finding him before Akihiko Sanada does, she would take the dog in with them and heal it using her own powers. Koromaru, faithful to Chidori and very much a mood lifter for all of them, was fitted with a special Evoker they created and given the best care that Minato could afford.
Minato would also infiltrate Gekkoukan High under the alias "Ayumu Ishikawa," making sure to be in none of the classes SEES were in. It's there where he would obtain most of their weaknesses and learn about them. Chidori and Jin were, however, bored by the idea, and decided to stay behind. As Chidori generally functioned as their support, she would still, as in canon, eventually meet Junpei and think (mistakenly) he was the leader. The rest happens as it does in canon, except that Minato and the others, of course, couldn't bear to leave Chidori in enemy hands, and after some scuffling, manage to get her back.
It is also worth noting that Takaya reappears, still kills Shinjiro who dies to protect Ken, and that Minato would happen upon the scene and take Ken back with him, shocked and angry. Strega also makes money by selling Suppressants among other illegal things, and particularly Shinjiro Aragaki had always been one of their best clients. (Naturally, Minato never took Ken out during these times, and never told him.)
Minato would also see Pharos as well as Minami during some nights--the same as in canon, with minor differences of being left cryptic hints of how there is "another". The canon point he is from is currently before the last Arcana Shadow battle.