User blog comment:B214/Question and Answers 3/@comment-4547267-20140624234231/@comment-24866657-20140625002153

Mostly they would try to stop him or talk him out of it first, Issei's not someone capable of lying to his harem with his whole heart, if they can't persuade him then Akeno, Xenovia and Ravel would like the ones follow him. Akeno because she's the closest to Rias, Rias would trust her to take care of Issei and continue to persuade Issei or try to find out why Issei decided to go evil; Asia wouldn't join because she's too weak on herself; Xenovia because she simply trusts Issei enough to go wild with him, and with less girls chance to have sex with Issei then slowly convince him is an option; Ravel because in 3 of them she would be the one be able to contact the Gremory or Phenex, she also would do her best to make Issei avoid bad decision in his evil career, for example instead of killing a high-class devil and his peerage, Ravel can manage to have Issei avoid that mission by doing something else like taking information, she also joins Akeno in persuading Issei and find out the reason.