Board Thread:TV Show Discussion/@comment-26510044-20150618214056/@comment-28600204-20150618235543

I think it's whoever's at the top of the chain of command who can legitimize, Roose requested that Tommen legitimize Ramsay and he did.

Stannis does not answer to Tommen and claims he is the rightful king meaning although weaker as he's not on the throne he could legitimize people although I'm not sure the rest of country would have accepted it unless he had actually claimed the throne later.