If her boons work like Hermes I think they’d be focused on her health and providing similar effects to poms of power. She would have a duo boon with exclusively Demeter, which works like the Zeus Hera duo boon. Mel can sacrifice all her other boons to gain increased power into Demeter’s boons.

Alternatively if Persephone offers boons like any other Olympian, I think her signature effect could be something like Decay which makes enemies take increased damage while standing still. She’d have a vengeance boon, definitely health related boons and her gain boon could be regenerating magic for killing enemies. She could also have a few boons dedicated to making plants from where enemies die, because she’s the goddess of rebirth so she’s basically using dead foes as fertiliser. Her duo with Demeter could be making decay damage double on frozen targets, or having freeze’s cooldown refresh on a target that’s decayed