During the events that led to the revelation of Akeno's identity, she falls in love with Issei after the latter says that he hates Fallen Angels but he likes Akeno.
It is also at this point that Issei becomes a source of comfort, as his kindness and courage helped her to overcome her hatred of her Fallen Angel blood.
After the events, Akeno has constantly been calling Issei "Darling" and started behaving like she is already his wife, meaning she accepted his proposal and she becomes Issei's second bride.