Gah. Coherency. Use better sentence structure please. >_<
Impa is not a Sheik in Skyward Sword. Impa was never a Sheik to begin with. Sheik is how Zelda hides from Ganondorf. Impa is Zelda's protector. Their roles are unique. Gameplay-wise, I don't remember Impa being just a Sheik as is. In fact, I remember them being different as is. Sheik seemed to have little magic overall besides the ability to teach it, but so could Malon... Impa, from what I remember, has a lot of magical powers. I know that Sheik technically has some magic, but that's because she's Zelda. When does Sheik show off any real magical moves?
Now, I know haven't played SS, but people can tell you that Impa has more magical abilities shown instead of "maybe she does" at best. They have the Ninja Vanish and both are based upon Ninja in common. I really wouldn't call them similar at all by this point.
Tingle is indeed hilariously awesome.
Skull Kid is different, but he feels closer to Yoshi to me. He's a significant version of one, especially in MM. He plays a role to the main character similar to how Yoshi does to Mario. Obviously, there's differences here and there. I don't consider him nothing but some generic guy. And Skull Kid is the main villain of one game. That's pretttttttyyyy significant.
Midna I did forget to mention, but I gave reasons to not have her before. She's connected to Wolf Link. I'd like no more than two playable Links in any form(or at least two slots max with a playable Link each. I'd be fine if Toon Link had Masked Link instead, mind you).
Ghirahim is rather neat, but you missed my point with Demise anyway. He'd be fun to add to Ghirahim's moveset.