This question brought me to an interesting concept....
Wouldn't it be cool if at the Final battle w/ the SK, their 'identity' was revealed....
What if they were revealed to be one of the defunct faction leaders from FE???
Or what if they were, in fact, one of the leaders from one of the rivals???
Then, depending on who the SK was, they may have a slightly different set of attacks/spells/guards in the final boss battle...
ahh, wouldn't that be cool?????