They really missed the mark with Revenant gameplay wise. Her gimmick is that she's supposed to be the summon class but her summons are borderline useless against Nightlords, at best they're fodder to tank normal bosses for like 3 seconds. Then they probably realized that her summons were so weak that they had to make her into a caster, but just forgot to give her a way to get mana back. They should have gone all in one direction, either make her a full caster or a summon based class, and since Recluse is already the dedicated caster, Revenant should have been entirely based around her summons. For example when you summoned one of the ghosts they each had a different moveset and you could play AS them while your character turned into a ghost, becoming invulnerable and playing the harp somewhere as if she was controlling them. Each summon would have a different ultimate and you could turn into the summon that had the best ultimate for that specific situation, one of them could revive allies (as the current one does), one of them could do damage, etc.
The issue is that this would make weapons (and equipment) useless on her but they could think of a different loot system for this character alone. Either way, they could have done something really different with this character.