Rias is the woman who reincarnated Issei and the woman he loves.When she first summoned him, she treated him as a "special" case, knowing that he had a lot of potential.They both passed and although Issei remained oblivious to the situation, he had mentioned that he would like to marry Rias one day.
She was also so heartbroken when she thought that Issei had died that she disconnected herself from reality, remembering only the times that she spent with Issei, and cried terribly with her father as he comforted her.
She, along with everyone else, later regained hope after finding out that Issei's spirit was alive, and she embraced him when he returned, begging him to never leave her again.
When she gains her derogatory nicknames such as "Switch Princess" (Bikou) officially used in Issei's "Oppai Dragon", an underworld's children's television show and "Crimson Bust Princess" (Azazel), Even though she hates those nicknames, she doesn't mind using her breasts for Issei's power ups, as she wants to be the source of his power and doesn't care if he is the Oppai Dragon.
In Volume 17, their relationship has improved to the extent that despite going out on a date with someone else, Rias chose to believe in Issei and the fact that he will still love her.
In Volume 23, Issei proposes to Asia, promising to make her happy and be by her side for eternity.
When Akeno first met Issei, she thought of him as a cute kouhai with whom she could spoil and flirt, but as time went on, she started to see that Issei was very kind and caring, eventually falling in love with him in Volume 4.
During this time, Issei was struggling to overcome the trauma that was caused by Raynare.
This trauma is the reason why he could not believe that the women around him would actually be attracted to him.
Issei is very protective of Asia ever since he met her, pitying the girl's suffering and was greatly saddened when she was killed by Raynare.