When a seemingly average character is secretly much more powerful than they appear

(Spoilers for The Witcher 3, Dragon Age Inquisition and Tehanu)

1 - Gaunter O'Dimm (The Witcher 3): we first meet him in a village, where he has a single scene in which he gives us info on Yennefer; cut to the Hearts of Stone expansion, we find out he's actually an interdimensional being with near god-like powers.

2 - Solas (Dragon Age): he's one of the starting companions and, while knowledgeable, he appears to be just a wandering apostate (a mage with no affiliation to any "official" institution). At the end of the game, we find out he's not only a godly figure of elven legend, he's the one who created the Veil, the barrier that separates the material and the spiritual planes.

3 - Therru (Tehanu, Earthsea cycle): she's a side character for most of the book, a heavily scarred and traumatised child that the protagonist, Tenar, takes care of. In the end, she's revealed to be a dragon born as human, and she summons the eldest of all dragons (who also happens to be Segoy, creator of Earthsea) to wipe out the villain.