Diodora was the reason for Asia's excommunication from the Church, as she healed him, a Devil, which was considered heresy. He appeared grateful to her to the point of proposing to her. Despite her rejection, he persistently tries to win her over, even attempting to swap Bishops but to no avail. While Asia stated that she doesn't regret helping him, she grew to resent Diodora for insulting Issei and slapped him for it. It was later revealed that Diodora set up Asia's excommunication in an attempt to make her a part of his "collection".
Issei disliked Diodora from the beginning as he was trying to "take" Asia away from him. Diodora, in turn, looked down on Issei, referring to him as a "filthy Dragon" and getting in his way of acquiring Asia.
Diodora had formed an alliance with Shalba in the Khaos Brigade, even handing Diodara one Ophis' to augment his abilities. However right when Diodora desperately begged for his help, Shalba murdered Diodora when he outlived his useful, viewing him as nothing more than a pawn in his plans.
While Freed and Diodora have never been seen interacting with one another in the series, Freed has displayed great knowledge on Diodora's plan to have Asia excommunicated from the Church.