Occult Research Club
Issei Hyoudou
Asia first met Issei shortly after Issei reincarnated into a Devil, viewing Issei as a kind hearted 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.
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 by believing that Issei's spirit was still alive and that he'd come back alive one day. Asia has even mentioned that she would like to bear Issei's child in the future.
Rias Gremory
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 and stated in Volume 9 that she wants to have kids with Issei in the near future. 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.
Koneko Toujou
Asia and Koneko appear to get along well, as they did a number of jobs together.
Xenovia
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, since then, everything has changed for them 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.
Irina Shidou
Asia, Xenovia, and Irina are called the Church Trio by Issei because they are often seen praying together. Asia considers her a great friend. Asia also sees her as a love rival and fights with her, along with the other girls, for the Issei's affections..
Fallen Angels
Raynare
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.
Other Characters
Freed Sellzen
At one time, Asia was an ally of Freed after she was banished from the Church. Even so, Freed was abusive towards 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.
Issei's Parents
Asia has developed a relatively close relationship with them and has seemingly been adopted by the couple as surrogate parents (and future in-laws).