Another option would be Fall-From-Grace, from the Planescape: Torment canon. As a succubus, she is naturally resistant to magic. As a chaste godless priestess who could be described as more of some sorta weird worshiper of the concept of belief, she can prepare a set number of cleric spells that can buff people's general ability to resist harmful effects, heal wounds/lost limbs, and resurrect the dead. Since she is already an extra-planar outsider who has seen some sick shit in the Abyss before she was sold off to the Baatezu, she probably has a degree of resistance to otherworldly influences. If we are allowed to accept 3e+ revisions to the succubus race, she can potentially gain several more racial abilities/resistances relating to psionics.
She's also very wise and has spent most of her time in Sigil as a sort of counselor; this means that she is probably has a lot of experience dealing with a wide range of beings from different planes of existence. Also, she can probably act as a ship counselor to help even people out if they're on the brink of freaking the hell out.
However, while she was able to give sage advice to the Nameless One despite him probably having a maxed out wisdom stat (thus indicating that she can solve a problem from weird angles), she is not a detective, nor is she an investigator. Also, she's not very durable. She may also stand out in a crowd without magic disguising, and will certainly show up on demon detection / evil detection due to her nature as a Succubus regardless of her alignment.