Eldrick is an important character in Desecration of Wings, another historian working with Felana. He is the most important NPC in the game - see below if you don't care about spoilers!
A calm but somewhat hesitant man, Eldrick seems to have deep reservations about many things. He's had a long relationship with Felana, mostly academic, and let things become intimate but not physical. Though he's dealing with some unique issues, in the early scenes with him you see how he tries hard to communicate honestly.
In reality, Eldrick is one of the abominations from the other side of the wings. He inhabits the shell of the original Eldrick, who meddled with the barrier and destroyed himself. The "real" Eldrick (his true name unfathomable to human minds) stayed in his body as long as possible to experience the world and be with Felana.
The real reason for Eldrick's hesitance was that too much contact (or even emotion) would destabilize his body and reveal his true form, which could only exist in the normal world temporarily. Plot events require him to show his true nature, which is a key component of the final part of the game.
In his true form, Eldrick is still sane and fairly human, able to form relationships with the main cast. Obviously his closest relationship is with Felana, as she is indifferent to the fact that he is a huge tentacled abomination.