Until now we have noticed that the only one who knows the truth about Jon Snow's real parents in Game of Thrones was Ned Stark.
Are there any others in-universe who know Jon's parentage?
Howland Reed was one of the few people Ned trusted with helping him free his sister from the Tower of Joy. Besides Ned, Lord Reed was the only survivor of that fateful day, which means he knows a secret that now only one of two people in the entire world know: Jon Snow is actually Jon Targaryen (though still a bastard, so he's Jon Sand).
Bran only knows about this lineage due to his warging ways, but Howland Reed was actually there and there's no way Lord Eddard Stark could have talked his way out of that one after going into the tower empty-handed and coming out with a baby.
There are hints that Melisandre might at least suspect Jon's parentage when she speaks to Daenerys