How do I deal with a PC with an NPC's soul bonded to them? [closed]

Question

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.

More info

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.

Answer 1

The "uncontrolled gestalt"

Something many, many players and characters find interesting is the chance to occasionally use an ability from outside their purview - especially when they're a non-magic type getting access to occasional magic. Let the players bearing pieces of the McGuffin occasionally access the shard's power to do something they can't normally do - rarely (1/day at most, probably) and with specific limits to what the shard can do (pieces of a sword should typically have martial-type powers, except where the sword has a specific reputation, such as a sword that sunders magic being able to dispel or a sword that breaks all swords that cross it being able to shatter.

Then, as soon as you've finished deciding on these rules, promptly break them for the character that's got the soul-bound shard. One of the benefits of an "intelligent item" is that it tends to be much more versatile than an ordinary magic item, at the drawback that the intelligent item has to want to help you before you can use its powers. Perhaps instead of a short list of sword-themed powers to choose from, this particular fighter can use one class feature of a Magus half his level once per day. Give him no control over the spells prepared or arcana chosen- the girl's class features known belong to her - but let him pick which one he uses when. Spellstrike to deliver a shocking grasp out of nowhere when you're a single-class fighter is pretty nice; the next day, the same ability could be used to spell combat in a spider climb without spending a whole round drinking a potion. On another day, he might call on the girl's arcane pool to get a key ability added to his weapon for a minute or two - guess who just became the party's source of fire damage when that troll just won't stay dead?

But make him work for his expanded freedom; the girl has to give him access to these powers, while the other PCs can claim their artifact-given powers any time of the day. Maybe she won't attack animals, because she thinks they're cute; maybe if they were arguing just before the battle (likely, given that she's basically responsible for the hero's generically tragic backstory), she'll refuse to aid him until he admits it's not all her fault. Maybe, despite her father's death, a future Big Bad will still be related to her (brother or sister - or mother - looking for revenge for daddy's demise?), and she'll hesitate to aid in their destruction unless the PC agrees to take him in alive - and convince his party to do so. Eventually, the two should learn to get along and he'll start to think of her as a part of his character, rather than a haunting spirit - not just for the mechanical benefits she provides - and then the eventual day when the sword must be reforged will either have to wait while the character finds a way to keep the girl around or be heartfelt and an amazing climactic moment as the girl's soul moves on.