Adamant seems best for Incineroar, but Decidueye was built to be a mixed attacker. Depending on what you want to boost and sacrifice, I would recomment the following natures for Owl Friend:
Lonely: More attack for less defense (its defense is lower than its special defense, so take that for what you will)
Naughty: More attack for less special defense
Mild: More special attack for less defense
Rash: More special attack for less special defense
Hasty: More speed for less defense
Naive: More speed for less special defense
Spirit Shackle is a physical move and its special movepool is actually pretty shallow, so we might want to go with an attack-boosting nature. Also keep in mind Decidueye can learn both Swords Dance and Nasty Plot.