Yesterday I got interested in creating a Duke. So I want to know...what the better build for them: AP, HV, HP or AP/HV?

i dont kno too much about dukes, but i would say first off that you shouldnt go AP/HV because they are two almost separate attack skills. Furthermore, Dukes are pure 3rd jobbers? i dont really remember names, but anyway if it is pure, you should really go for full HV because most skills revolve around that stat for pures. whereas a hybrid third jobber would do better with AP skills because cats generally have more skills that revolve around AP.

Pure HV is good and common choice if you want a powerful Duke. Notice that your greatest power lies in Metamorphosis and Wild Nail. ^^ HV character is a bit pain train at first but in the end it's worth it. I have Duke myself, pure HV. Actually, leveling and playing can be a bit (or more) boring until you reach 3rd job... At least I got mouth, ears and nose full of Card Strike, as it's actually your only attack skill until 3rd (Galder Thrower doesn't count, it really loses its meaning soon... Keep it for Giuvanni and 3rd job trials though.).

But be patient! It's worth it: Wild Nail does awesome damage and you'll get Volley Kick: It's faster and more powerful than Card Strike (at least with pure HV build). Also, you get all possible defence buffs: Duke's path opens door to Sumo Suit and Team Bolster. You'll be The Tank with buffs and Meta. ^^ Also, get One Pair to be good fighter outside Meta too. 

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Xandrielle ~ Primadonna 18x
Merfialna ~ Soul Master 16x
Elycia ~ Thief Master 13x
HikaruMinato ~ Duke 14x
Nyreem ~ Boxer 11x
DarAnovel ~ Magician 7x

Jewelia/Pearl~