Hanya ideas

1. She can cast Impa's Disguise and Vanish with Weskin's MP? It's a cute thought, but Roscura can disguise all of us already and that'd be six tokens for six tokens' worth of spells.

2. Some sort of special interaction with Ancient technology? Messing with old consoles that'd normally need a Sheikah Slate? Probably nothing as drastic as temporarily possessing a defeated or inactive Guardian but... well, actually something in that vein might be kind of thematic considering Spirit Tracks/Phantom Zelda. That's probably way too much though. Oh god, Rei said they could possess undead bodies too, didn't she.

3. SPIRIT SHURIKEN??

4. She empowers one of Weskin's weapons, becoming its weapon Spi- HAH, just kidding I'm not going to try and squeeze that out of you. There is no 4, Sheikah are superstitious that way.

5. Use certain combat maneuvers on a marked enemy as a Minor action through her? (Like Coordinate or Predict, or... I guess that's it, really. Weskin's kind of bad at those maneuvers though)

6. Inverse fairy - can create an area of darkness around herself and cast Vanish?

7. Leftover power of the moonlight infuses her. Bonded to her, Weskin can finally see in the dark and ????

8. Blue fairy focus, but some sort of extra benefit when using Sheikah style weapons? Deshoo can make Sheikah style weapons when Hanya is around??

9. Maiden's Rally - When an ally within (x) is healed by magic, heal them (x) more/grant them an acc boost to their next attack. IDK I'm starting to reach here.