I’ll be the one to say that I’m not 100% on Fugo’s backstory.
On one hand, they added more details to a rather ambiguous plot of Fugo that was to be later elucidated on his own spin-off novel but I feel like that said ambiguity in Fugo’s past only added to emphasize his unstable temper. Back in the original publications, the tidbit of Fugo’s past that was of interest was that he beat his instructors with a heavy book during his school years which caused him to be ostracized which led to where he is now. The anime however added a touch of pedophilia to the instructor Fugo straightened up so it makes his anger justified, although still reprehensible by law. I don’t quite agree with this sort of characterization since Fugo was completely in the right to beat up his instructor while the off-shoot novel made it so that the instructor made fun of his grandmother who he respected which triggered the beat down. I definitely prefer the violent outburst that way with an insult taken too far instead of Fugo directly trying to defend himself since that drives the point home on his inner violence compared to the justified decision Fugo got to doing while simultaneously being accosted for it.
As per orders the trio Giorno, Fugo, and Abbacchio head towards Pompeii to retrieve a key located near the dog mosaic in the ruins. Fugo has a fit of anger but suppresses it against Giorno giving directions while Abbacchio still has doubts over Giorno being part of the team. Nonetheless, the trio make it to their destination where Fugo finds a strangely placed mirror that conveniently already has a suspicious figure watching their move from a pillar that’s behind them. Fugo notifies his comrades about this but they find Fugo’s words hard to believe as there’s only a single pillar and nobody seems to be peering at them behind it. Fugo turns back to the mirror to clearly see a figure walking towards them with no intent on hiding himself. Fugo yells at his crew to get ready but yet again, they turn around to see absolutely nothing. The man reflected in the mirror closes in and reveals his Stand and Fugo pushes the unknowing Giorno and Abbachio away and Fugo finds himself alone.
Our next Assassination Squad hitman is Illuso with the aptly named Stand, Man in the Mirror whose effects are deduced by Fugo as he finds his watch on his opposite hand. Illuso comes clean with the fact that Abbacchio and Giorno weren’t the ones dragged in but that it was Fugo alone who was separated and entered into Man in the Mirror’s mirror world. Illuso explains his ability to control what enters and leaves the mirror world and that Fugo will be trapped in here to be killed alone if he does not give away information and of course, Fugo refuses to cooperate. Man in the Mirror approaches for the attack and Fugo sees no other choice to unleash his own stand but finds it unresponsive. While he feels that it was brought out, it is nowhere in sight and back out in the real world, Abbacchio tells the investigating Giorno to move back. Abbacchio yells at Giorno as Fugo’s stand, Purple Haze, appears in front of them. Giorno notices Abbacchio’s concern for their safety over Fugo’s and his suspicions are answered when Purple Haze punches a wall and Abbacchio makes an even stronger point about creating some distance between them and the stand. He explains that Purple Haze’s fists have bulbs on them that contain a deadly and airborne flesh eating virus that rots their victims from the inside out. The virus does not distinguish between friend and foe and annihilates anything within its effective range. While it neutralizes under sunlight, its effects are still quite lethal and Abbacchio shares his intent to get to the objective first. Giorno rebukes this and says that they should rescue Fugo first before getting to the dog mosaic. This is around the part where we get Fugo’s backstory but I already went over the important stuff above.
Abbacchio and Giorno reach an impasse where the former simply states that he would have preferred Fugo and Giorno to abandon him if he ever was caught in Fugo’s situation and leaves to the objective, letting Giorno do what he wills. As they part paths, Purple Haze destroys the wall holding the mirror Illuso used and while Abbacchio thinks it was just a result of Purple Haze’s random violence, Giorno tries to see if there’s a meaning to the destroyed mirror.