Howland Reed is still alive (The show haven't shown him in present day yet, but he's still alive in the books, so until the show tells us otherwise, he is still alive on the show as well)
MysteryXX is right, if it hadn't been for Howland, Arthur Dayne would have killed Ned. It's the same in the books.
He is probably the only living person in the aSoIaF universe who can confirm Jon's parentage. Because he was with Ned in the tower, he didn't stay outside