Tbh, It's baffling to me how underrated the Frank and Percy friendship is. Frank was the first person willing to vouch for Percy(even before Hazel) when Percy first came to CJ.
Percy and Frank adore each other. They both think the other person is awesome due to their powers and skills. Frank feels secure with Percy so much so that he opens up about his mother's death(a topic that isn't easy for him to discuss).
Even the goddamn pjo wiki lists them as family and states that they 'have a very brotherly relationship'. Yet despite all of this Rick(yet again) rarely writes about their bond in the later books(aside from Son of Neptune). Plus the fandom often forgets about them.