Why was Shiek still being so mysterious?

It had nothing to do with sheikah culture Link had checked with Impa and Zelda on that one. Nor after Ganon was defeated.

Plus Sheik had promptly creamed Link in 20 different card games, with the bet of removing either Link's hat, Sheik had walked around or teleported Link still wasn't sure witch, all of hyrule wearing Link's hat, (Link had lost, badly). Or if Link won Sheik would take off his mask and hat.

When he asked Zelda to order Sheik to remove his scarf, Sheik had waltzed right in without it.

He had the mask of truth on however.

Even mentioning that Zelda and Sheik were the same person could get you killed. Sheik was very vindictive, and those coccos were no walk in the park. Fortunately Link had a fairy at the time. Otherwise he wouldn't be standing at all.

It was extremely confusing, and Link was very persistent.

Even if Sheik caught all the big poes (it really wasn't fair, where did he put all of them?) waited till he was asleep before releasing them into his room.

Sheik was also a secret arsonist, Link found that out after his tree house went half up in flames, at the time Zelda wanted him at the castle and Sheik had some prior engagement, so Sheik arranged for all the inns to be full, with Link left with no other option of having to stay at the castle and protect Zelda from more kidnapping attempts.

Sheik could have just changed the guards around but noooooo, he wanted to make Link's life miserable.

And why couldn't Sheik just defeat Gannon on his own without having to wait for 7 years till Link showed up?

Nope, and Link was determined to find out why.

Finally Link found the Sheikah standing on his head, and it gave him a malevolent idea. He ran off and frowned his prey by the new deku sprout. Stuffing it into his especially prepared trap he quickly left the forested dominion. Jumping onto Epona, who was quite used to Link's attempts at racing around to help others. Link whispered his plan to his captive promising obedience from the new one.

The malevolent force of picturesque innocents agreed. Wondering the same as the hero.

Any attempt at night would have Link thrown out of the castle. So Link just walked up to the gate, used a small minuscule amount of power from his secret weapon, Link had intended only one guard to fall, all the guard did, so potent was his secret weapons tenacity at being used thusly.

Link skipped up the path to the castle, across the moat and over into the outer area of the castle. He stopped by Zelda's room yesss, his prey remained unaware lurking inside.

Link smiled as he entered, "Hello Sheik."

Sheik raised a baleful eye at the hero, before it widened slightly at Link's secret weapon, before Sheik could escape, Link unleashed the contents of his bottle.

Sheik writhed in agony.

Link found the distress of his comrade fulfilling and made a memo to himself to check out the new postings of geroudo king. Sheik with all his sheikah training and iron will took an astounding amount of time to surrender, a whole minute!

Link was impressed, "So Sheik want to answer my question?"

Sheik nodded still shaking from the torture inflicted.

Link caught his nefarious minion and stuffed her back into the bottle.

"So, why are you still being so mysterious?"

Sheik gulped, at least Link thought he did, and mumbled something, "Sorry I couldn't catch that."

Sheik glared at him, "I said I got really bad sunburn waiting for you to show up at all the temples."