In the third chapter of Daniel, 3 Jewish men get thrown into a fiery furnace. The King who had them thrown in looks and sees a fourth person walking around with them in the fire. This fourth person is identified by the polytheistic King as looking like unto a son of the gods:
He answered and hath said, 'Lo, I am seeing four men loose, walking in the midst of the fire, and they have no hurt; and the appearance of the fourth is like to a son of the gods.' - Daniel 3:25 (YLT)
The men who were thrown into the fire had previously claimed faith that God, if he so chose, could rescue them from the fire:
Lo, it is; our God whom we are serving, is able to deliver us from a burning fiery furnace; and from thy hand, O king - Daniel 3:17 (YLT)
It is likely that this confidence was built in part upon a direct promise from God issued in Isaiah 43:
And now, thus said Jehovah, Thy Creator, O Jacob, and thy Fashioner, O Israel, Be not afraid, for I have redeemed thee, I have called on thy name—thou art Mine. When thou passest into waters, I am with thee, And into floods, they do not overflow thee, When thou goest into fire, thou art not burnt, And a flame doth not burn against thee. - v. 1-2
The promise is that in the fire they are not burnt because "I am with thee". Jehovah uses a first person singular pronoun. Later in Isaiah 43 Jehovah makes it clear that He alone is Savior:
I—I am Jehovah, And besides Me there is no saviour. - v.11
I understand the notion of proxy, and that God could still be considered the only Savior even though sending a Savior who is not Himself, but the promise in Isaiah is that God Himself with be with them in water, flood, and fire.
Whom do Biblical Unitarians say is seen by Nebuchadnezzar in the fiery furnace with the three Jews?