For me I think X-1 Mask is the first extra hero even though he was only in one episode. His powers were destoryed in battle, but it doesnt mean it can still be around. I mean if Dekaswan counts and she only had a fight in one episode why shouldnt he count. I think if there is a key for him it will be very interesting or the original actor appears for the Maskman tribute.
I know they arent all extra heroes, but I just think that some of these characters that dont have keys (yet) should be known as extra heroes.