How important do you think the Sheikah truly are in the Zelda lore?
Well, you're really asking
two questions there.
The first takes me back to nineteen dikitty eight. We had to say dikitty because Ganondorf had stolen our word ninety.
Nah, but really I see this as two question: how important are they to Nintendo and how important are they to me.
If we're talking about Nintendo I'd say not very. They've been in three games (Ocarina of Time, Twilight Princess, Skyward Sword) and the Lens of Truth showed up in Majora's Mask so maybe you could count that, I guess, sort of, if you want, kinda. Veran in Oracle of Ages had the eye symbol on her, too, but beyond that there's nothing. Sheik was originally planned to be in Twilight Princess but was cut for whatever reason, and, like Spiritual Mask Salesman said, we don't know much about them beyond some basics.
Nintendo seem to bring them out when they want someone to protect the royal family in some capacity and an army isn't going to do. It seems to me that they're just one of those bits of lore that Nintendo has 'in the bank' and they pull them out arbitrarily when it suits them. I'd say a case can be made that the only reason Impa was in Skyward Sword was because of the art style. I don't think Nintendo think the Sheikah are very important at all.
If we're talking about me I'd say they're quite important. I love the Sheikah. The Sheikah are one of the things I love most about Zelda lore. Yes, largely because ninjas are cool, but also because I like female characters who are ninjas, I like disguises, I like when people drop a flashbang and then when everyone can see again the ninja is gone and everyone's like 'Whaaaaaat?!?!'. But I also like the mystery.
It's like the dwarves in The Elder Scrolls. They were a major race, building robots and all that, being hated by everyone because they're a bunch of Richards, and then one day ...
poof! They're all gone. Just gone. Where to? Who knows. And
that is precisely why everyone is interested in the dwarves,
that is why so many people say they want a future Elder Scrolls game to bring them back.
The Sheikah are like that for me. There are so many unknowns that I naturally want answers, but their nature of being deceptive and stealthy and keeping secrets adds to that mystique. It creates the impression that I don't know these answers because they don't want me to know, not that the answers just don't exist because Aonuma is too busy sculpting his beard into middle-aged perfection. I like Zelda theory. I like trying to connect the dots. I like extrapolating on small details and seeing where they take me. The Sheikah don't show up often, but they show up often enough for them to be a constant thought in the background any time I'm speculating.I like the idea of a benevolent shadow organisation working away unseen throughout Hyrule's history. It helps fill in a lot gaps in a plausible way. Zelda lore wouldn't be as interesting or as fun for me without them, so, to me, they're very important.