An unidentified friendly male voice that talks Carl through the psionic assault and calls him family.
At the depth of the psionic barrage, a voice reaches Carl that is not Shi Maria and not her husband. It is familiar, male, friendly — a dwarf, Carl thinks, along with a memory of tendrils worming their way through his body. It tells him to steady himself, that he has done it before, and that what they think is his biggest weakness is the opposite: his mind is the rock upon which the rest of him is built.
When Carl says he is going crazy, the voice answers that this is the second time it has told him he is not, and that he is family now, after what he did. It directs him to look at his feet, where he has been standing in The River this whole time. After Mind Balance hits 16 it says "Good, Carl. Good. She would be proud of you." Carl asks who, and snaps awake without an answer.
The unseen ally whose intervention pushes Carl's Mind Balance to level 16 mid-battle.