Agree, compare to others crown prince he actually good but lack of experience maybe bcs he crown prince so he not like us who can I have many type of friends who can teach us something. He actually did his best to solve thing up between him and Diana but Diana just too stubborn.
And for reason, in my honest opinion. I think why he like his friend wife bcs what I said before lack of friend, and hestia come to his life just remind me of theory of 'we Fall in love with 3 people in our lifetime's where hestia maybe his second.
diana is too stubborn indeed, he's not blameless but their relationship failed mainly because of diana and her pride, if diana had acted like a real crown princess maybe their main issues would have been resolved, and i think that's why he likes hess, she loves her husband and also acts accordingly to her position even though she was originally a commoner, and i didn't know about that 3 person theory, but would make sense in helios' case or at least i hope he gets someone better than diana (as hestia won't look his way EVER)

helios is not that bad of a character, he's actually a good ruler but is so weird that he's got interested in his bff's love interest TWICE, just as weird as his wife