Léahs power in lore is mainly aether siphon and transforming his limbs using root-like strutctures to do it better. I based this stupid power of his from the pokemon move leech seed and the imagery of ivy plants depriving a host plant from nutrients

Which in turn i think combines well with thiccolas being a vampire… he needs sustenance through blood and leah through magic which they take from each other… mutually parasitic romance…..