Guide Mentor
Character Role Analysis
Randolph Henry Ash, Christabel LaMotte, James Blackadder
Randolph Henry Ash doesn't actively mentor Roland Mitchell (he's way too dead for that), but he does serve as an intellectual and artistic role model for his twentieth-century counterpart.
Just as Randolph Henry Ash serves as an intellectual and artistic role model for Roland Mitchell, Christabel LaMotte serves as an intellectual and feminist role model for Maud Bailey. Although Christabel doesn't actively mentor Maud (again, these nineteenth-century characters are way too dead for active guidance), she's an important figure in Maud's life all the same.
Of course, it wouldn't be fair to exclude James Blackadder from this list of guides and mentors, even though Roland Mitchell tends to think of his former supervisor as a discouraging and not entirely helpful figure in his life. As Roland learns by the time Possession draws to a close, though, Professor Blackadder really has been looking out for his best interests, and has done all (or at least most) of the things that a good supervisor should do.