Brief version: an NPC's soul has just been bonded to a PC, but I haven't decided on the mechanics of what that means yet. How do I do this in a fun way?Background
My party is currently on a long-term quest that involves gathering the pieces of a magic sword (of plot advancement). Each piece that they collect appears as a tattoo on a particular PC's arm. The most recent piece is the first they've found that was previously bonded to someone else, in this case the preteen daughter of the late (former) Big Bad. After taking it, the girl fell into a coma; they've left her in the care of a priest, and are now getting on a boat to go off looking for the rest of the sword.Problem
The players don't know it yet, but when they took that piece of the sword, her soul came with it. I haven't decided yet the effect that her presence is going to have, though she hasn't done anything yet in about four days of game time (mostly travel).
How can I move forward with this in the most interesting way? The party has other goals to accomplish, but I'd also be okay with this becoming a substantial component of the game for a while as long as it's done in a way that doesn't marginalize the other party members too much. It's also maybe important to keep in mind that it's not like the PCs should have expected this (and, honestly, I only decided this is what would happen after it actually happened) – so if they're not into it, it should probably be either ignoreable or resolvable.
One option is to have her "wake up", and then be a running-commentary voice in the PC's head (kind of how I imagine an intelligent magic weapon might work). This has the potential to be fun, but I can easily see it getting frustrating, especially because there are only limited situations where she could actually be useful to the party.
Another is to have her only subtly affect his emotions and/or dreams, which I'd just convey directly to the player. This would probably just be mildly intriguing and/or annoying until they figure out what's going on, at which point I don't see it really doing much other than a possible motive to free her.
A harder option is to work with the player to try to either actively merge the two characters' personalities, or have the personality swing between the two. I don't really see how this would work, and definitely don't want the latter's game effect to be along the lines of "be incompetent for a round if you fail a Will save in every fight." (We haven't seen much of the girl's personality, so I suppose she could just be badass too to avoid any huge combat effects.)
The minimalist option is that they're not directly aware of her presence at all; it only comes out later, when they make more progress with reuniting or understanding the sword. I think there's the chance for something more immediately interesting than that here, though.
Personality-wise, the PC is a gruff, awkward fighter (low Intelligence and Charisma but good Wisdom), 18ish, male. Both characters are human, but the party are all from a marginalized nomadic tribe, where the girl is from a wealthy colonialist family. Her father ordered the campaign-inciting atrocity against the party's people indirectly because of her; now that he's been stopped, the party is dealing with whether they want to start a revolution and try to kick out the oppressors.
As mentioned, the party had only minimal interactions with the girl so far, so her personality is still pretty flexible.
The player in question is not necessarily the most active roleplayer, but I think he'll be interested in trying out something new. If it turns out to be a disaster, it should be easy to introduce a side-quest to free her soul or whatever. I'm also not super-experienced at GMing complicated roleplaying situations.
If it's relevant, we're playing Pathfinder, online, with a homebrew setting.