Hazel is good friends with Frank in The Son of Neptune and even both secretly have feelings for each other. They are both outcasts, and have a lot in common. She thinks he is funny and even mentioned that he makes a handsome elephant.
During the quest to Alaska with Percy, Frank reveals that he knows of Hazel's secret that she came back from the dead. Both experience a blackout where they live through Hazel's last moments alive and her time in the underworld. Frank also tells Hazel about his secret, that his life depends on a stick.
After the battle against Alcyoneus, Hazel tells Frank of her affection and the two share a kiss, and thus begin dating. The demigod Leo Valdez, however, creates difficulty in Frank and Hazel's relationship. Frank is loyal to Hazel. However, Hazel is drawn to Leo because of his similarities to Sammy, who was Hazel's boyfriend in her previous life. This causes conflict between Frank and Leo and they even begin fighting.
Frazel is one of the most popular ships in The Heroes of Olympus. It is one of the canon ships and therefore has gained popularity and attention.
As of 2018, Frank/Hazel is the fifth most popular ship in the Heroes of Olympus fandom on Archive of Our Own.