Both Iris and Princess Hibana were present for the events of the end credits sequence when someone presumably combusted and burned all of the nuns and the church—except for the two of them. But while Iris’ faith in the Church of Sol seems to have strengthened since that tragedy, Hibana has all but abandoned hers, and has pursued a life of inhumane, heretical research.
The gulf between them is weighing on Iris, who wants answers but won’t tell anyone in her company, including a curious Shinra. That means Iris leaves the safety of Company 8’s station to pay a visit to Hibana at the 5th. The mere sight of Iris’ holy raiment enrages Hibana, and she burns most of it away, mocking all FFS nuns as mere “window dressing.” Shinra, Hinawa, Maki, and Arthur are quick to mount a rescue; hey’d been planning to raid the 5th anyway; Iris simply accelerates their timeline.
Hibana’s eclectic collection of pyro-weirdos don’t really cause that much trouble for the outnumbered 8th; one 5th soldier who blows explosive gum bubbles is outdone by Hinawa’s ability to control the speed and course of bullets from his guns, the “Three 5th’s Angels” are no match for Maki, and Arthur is able to deal with the souped-up captive Infernal when he realizes he was using his wrong hand. He’s an idiot, but a strong one.
All of this allows Shinra to slip behind Hibana’s defenses and reach the front door of her central mansion. Perhaps, when she’s rescued, Iris would be so kind as to fill in those who saved her on why exactly she did something so reckless as entering enemy territory alone, as well as why her smiles look so forced. The 8th is a family, after all; there shouldn’t be secrets.