birth father
I have not seen anyone else address this. From the movie Deathly Hallows Part II, I've always detected a feeling that Harry's real birth father is Severus. He and Lilly seemed connected and close and with his deep true love and concerns for her, at one point along the way I feel they got together.
From Snape's tear, Snape is crying holding dead Lilly - then skips to Snape and Dumbledore where Snape releases the doe patronus. Dumbledore says "Lilly ... after all this time"
Then at Kings Cross, Harry says to Dumbledore "my mother's partronus was a doe, same as Snape. I find that curious" and Dumbledore says "come to think of it, there's nothing curious about it"