Story line possession - ideas for body cohabitation

[Deadalready]

I'm running a module where in one of the rooms there's a lonely wraith who was apparently a consort to a prince but now roams the dungeon looking for companionship. The module says she tries to "caress the pcs of their souls".

Considering nearly all of the dungeon is fight encounters, I've decided to make this encounter into a diplomacy based one where the wraith wants a companion and the PCs have to weasel themselves a solution. However with each sentence the PCs trade the wraith, she moves closer and closer until she jumps into someone's body.

For the majority of the time I plan for her to be a passive observer in someone's body. What I do want is for a good bonus/trade off for a PC to gain having another spirit cohabit their body.

Honestly I'm not sure where I'll go with the possession just that it'll make an interesting personal story line for one of the players.

Suggestions comments?

[Ascalaphus]

Downside: while you're not being actively Drained, with an undead in your body positive-energy healing doesn't help you either.

Upside: you're under a continual Hide From Undead spell because all the other undead think you're one of them.

[Thornborn]

Upside: Spells targeted on you might instead target the roomate.