Rumia

Rumia is a unit that is obtainable relatively early in the game. She focuses on attack power but is more balanced than Marisa defensively. Her special status effect "Darkened" is applied to enemies after an attack about 30% of the time, and her level growth is fast.

Where to Obtain
Rumia and her spell cards are acquired from the Main Quest "Forest Behind the Shrine," which is available on Easy difficulty to units level 2 and up.

Strategy
Rumia's status effect effectively cuts any multi-hit attack down to one attack, making it an effective nerf for many spell cards where the overall damage is high but the individual hit damage is low, such as Meiling's Extreme Color Typhoon, Sakuya's Eternal Meek, or Tenshi's Sword of Pleasure after Hardship. Demarcation's passive effect will apply to both normal attacks from purple-aligned characters, such as Koakuma, and also to purple-aligned spell cards, like Silent Selene, Philosopher's Stone, and Young Demon Lord. However, if Rumia activates another spell card over Demarcation, its passive effect will cease, so it is important that Rumia doesn't use any other active spell cards if one wishes to see units other than Rumia applying the status effect. Demarcation's high amount of hits creates the highest chance to apply darkness, so it's the best active spell card to use if one is looking to take full advantage of this status effect. Rumia's "Darkened" status effect also stacks with Cirno's "Frozen" status effect, but the two effects don't synergize together in any notable way. Eating is a passive card that creates a battle state, which means it will not deactivate Demarcation if applied afterwards. Besides this, Eating is a card that allows Rumia to frequently attack out of turn on numerous targets, increasing the chance of darkness being applied to enemies. Eating also works well with Daiyousei's Call for Help as the fairies will add extra hits not just from themselves but also through triggering Rumia to do so.