If you think about it in the game the guild was distorted and it was said there were no more heros. Also Reaver made a deal with the shadow court before the purge. So that would mean that he could be the only possible father with the bloodline. Lastly you start the game in poverty. It would make sense if Reaver abounded the hero's mother.
Reaver is not part of the Archon bloodline, he is just a Skilled hero. If your going to guess Reaver as the father, why not Garth? The guild was purged, and it was believed there was no more Heroes, but they went into hiding. For all we know, the bloodline came to the Hero of Bowersone via their mother, much like it did for the Hero of Oakvale. We have no info about parentage for this Hero and so have no clue which side gave the Archon bloodline.