Roronoa Zoro. [manly nod] Sounds like a good deal to me.

[he can leave Luffy and Sanji to their own devices for a bit while he tails this girl, finds out what he can about ghosts, and keeps her from getting hurt. Shh yes he's a pirate who protects.]

'Sides. If it turns out you and your camera are the only things that can take down these ghosts, no one in their right mind would let you roam around alone.