Asia first met Issei shortly after he was reincarnated into a Devil, viewing Issei as a kindhearted person. After Raynare extracted Asia's Sacred Gear out of her body, Asia in her final moments as a human thanked Issei for being her first friend, devastating Issei until Rias reincarnates Asia into a Devil. Asia has strong feelings towards Issei and wishes to stay together with him forever. Whenever Issei thinks of anything perverted or behaves in a lecherous manner towards another girl, she would often get teary-eyed and pull his cheeks. Though she realizes that Issei is in love with Rias, she doesn't give up on her desire to be with him.
Since Issei is her first friend and the first person she has ever fallen in love with, Asia is very determined to win his affection. If another girl acts seductive towards him, she will often jump in, declaring that she will not lose. As she is well aware of his perverted tendencies, she will sometimes dress in revealing clothing to impress him. She also does not mind if Issei sees her naked, and has stated that she does not mind if Issei gropes her as long as he asks her beforehand.
When Issei died, Asia was in such a despair that she was thinking of killing herself to be with Issei forever, but stopped herself when she realized he would be mad at her. Asia regained hope after hearing that Issei's spirit was still alive, and she embraced him when he returned. Asia has even mentioned that she would like to bear Issei's child in the future.
Asia cares for Rias like a sister even though she knows that Issei loves Rias. She will not give up on her love for him, but still supports Rias' relationship with Issei, even helping Issei to confess his feelings for Rias. Both Rias and Asia also show the same interest as they both found kid Issei very cute and work together to turn him into a child for a day.
Asia and Koneko appear to get along well, as they did a number of jobs together.
Asia and Xenovia are best friends. When they first met in Volume 3 there appeared to be some tension between the two after Xenovia showed her disgust at the holy maiden who had been turned into a Devil. However, after Xenovia became a devil herself, the two became good friends after they spent some time together and shared some common interests. Asia and Xenovia are often seen together praying but Asia treats her as a love rival since Xenovia is often trying to seduce Issei into doing naughty things.
As both of them were raised in the Church, both are naive when in comes to sexual situations, and often take suggestions from their classmate, Aika Kiriyuu. Asia will often follow Xenovia's lead on seducing Issei, as she doesn't want to be left out.
Xenovia feels guilty of her treatment of Asia when they first met, becoming very protective of her. When Issei isn't around, she feels in charge of Asia's safety.
Asia, Xenovia, and Irina are called the Church Trio by Issei because they are often seen praying together. Asia considers her a great friend, but also sees her as a love rival and fights with her, along with the other girls, for Issei's affections.
Ophis and Asia are shown to have a good relationship. Ophis seems to get along with Asia the most out of the other club members, besides Issei, as she blessed Asia with the ability to communicate with dragons. Ophis described her as a sister with normal sized boobs and long gold hair.
Raynare took Asia's Sacred Gear but was defeated by Issei and terminated by Rias. Rias returned her Sacred Gear to her and reincarnated as a Devil to become a member of her peerage.
At one time, Asia was an ally of Freed after she was banished from the Church. Even so, Freed was abusive towards her and never showed any concern for her, and Asia feared him. He even took part in the plan of removing her Sacred Gear, knowing full well that the process would kill her. They have since become enemies with her transformation into a Devil.
Asia has developed a relatively close relationship with them and has seemingly been adopted by the couple as surrogate parents (and future in-laws).
Diodora was the Devil that Asia healed which resulted in her excommunication from the Church, although Asia stated that she doesn't regret doing so. He appeared grateful to her to the point of proposing to her, however It was later revealed that he set the entire thing up in order to make her a part of his "collection".