Kamihime are tied to their weapons. You get those weapons from the Jewel gacha in general, if you're lucky. The first time you get their weapon, you get the Kamihime. But not all weapons have Kamihime in them.

At the moment there is a Raid event going on, if you do that you get Event tickets. From these, you can win "Love Compact" SR weapon or something, which unlocks the SR Hime Cupid. The same weapon is also available from the Soul rewards of this event - either way you get it, you unlock Cupid.