I'd like to throw some counterarguments to these theory …
What if every time she creates life with Nii-chan as a conduit (he provides the earth) that she uses a bit of her own life force to do it. That would make things EXTREMELY dangerous for her to continue using them in such a way and being an Exorcist would quite literally shorten her life. She gets taxed far more than any other tamer when using Nii-chan to grow plants.
I’m positive her mom has ulterior motives, and this is a little out there, but maybe THAT’S what she was told to her and it scared her into quitting, on top of being HUGE info [Nephilim]. She’s too far removed from being a “living concept” (as Mephisto described demons) and is a flesh and blood creature, there is no endless source of power. She can’t generate life from nothing and it has to come from somewhere... herself.
And just like how Yukio was concerned that Rin was using his flames too much (at the cost of his humanity), her power use comes with a cost, too. Her life.
However, Shiemi was with Nii-chan when she made the decision to quit in chapter 87. Moreover, next chapter she even use it, not to save people, but to create a Christmas tree for the party
but unfortunately ruined the party instead. So I think it's highly unlikely she's afraid to use this power. And if it's anything about her health you would expect her mother to explicitly forbid her just like in chapter 3, but her mother was like “After hearing this, if you still insist I won't stop you.” Moreover, if it's too risky to be a tamer, she could just be a doctor or aria.
Like Cloelia said, it's quite common for a tamer to feel exhausted after summoning. Until now Bon, Shima, Izumo all have once lost conscious after using a huge tamer ability. (Though it seems like Shiemi is the most serious case due to the apparent nose bleeding, but think about it, she is confronting
Satan’s son.)
Anyway, here I am with a (not cracky) but totally crazy theory. Feel free to kill me, I allow you. But here's what's been running in my mind these last 3 days: what if it turns out Yukio, and I mean the Yukio we've seen so far, is actualy the demon version of himself while, opposite to Rin, is his human side the one "trapped" somewhere inside him?
Not cracky at all! I thought of something similar. If Yukio's powers were simply dormant, then they were never sealed, meaning that Yukio might not have developed a human soul like Rin. Instead, he was a demon soul without any powers in a human body. Because that soul developed over sixteen years, it's largely the same as a human soul. The only difference is his supposed demon's nature. I think this would explain why Rin's humanity was imperative to protect, while Yukio's was irrelevant. Yukio couldn't be possessed, but Rin can be possessed by his demon heart or by Satan if his demon heart is destroyed.
We have, in fact, seen Yukio's "human side" many times. The most noticeable one should be in Shura's arc when he realized he "can't let her die". There were also times like he was worried about the exwires' life when everyone was separated at nine-tail arc, and he saved the boy at the start of the IK arc. Not to mention his affection to Shiemi, which is definitely a human thing rather than a demon thing. IMO Toudou's words were about how you can manage to control yourself even though you have some dark thoughts in your heart, and Yukio already passed this test at IK arc
(about human possessing … it's just pretty rare in this series and Yukio is no exception. His pressure isn't necessary heavier than the various people in IK arc or nine-tail arc)
Kinda rather serious, but a theory for why Mephisto denied giving Yukio his answers is because Yukio was rather rude about it. He did pretty much order him for the answers in a really...snappy way. While Rin seemed, calm and collected about it, and rather nice about it as well from what it looked like. So all in all, Mephisto will give you what you want if you treat him nicely.
Yes... and I think the timing was also terrible.
Mephisto also was about to go on stage for an important speech right when Yukio asked. It was thoughtless on Yukio's behalf to confront him at a time like that. So of course the answer will be 'no, leave a memo!'
I strongly believe that Mephisto wouldn't have told Yukio a thing, no matter how Yukio asked. He's always done that. He smiled and brushed off Yukio's demands to know right after tossing Shiro's old job onto Yukio's shoulders. He taunted Yukio when he called in to ask about backup in the Inari arc, saying if he failed it was just because of his own incompetence. He is plenty aware of Yukio's situation, and only brings it up once in order to alert Rin.
Yukio obeyed Mephisto's orders loyally for months, initially trusting him, but gained nothing out of it. In fact, from his perspective, Mephisto simply exploited his loyalty. So why the hell should he be polite now? He was a loyal dog and it got him nowhere. Yukio also literally just came from of a freakin' suicide attempt. He completely, fully, and one-hundred-percentedly intended to kill himself. Needless to say, he wasn't exactly rational when he survived for reasons he couldn't understand. Yukio was in total rage, because nothing made any sense anymore.
Mephisto knows this. He knows everything. He knew Yukio was at the end of his rope. And he grinned that goblin grin, content to deny Yukio any answers. He was laughing at a ridiculous child who'd been stupid enough to think he deserved any answers.
I disagree most part, but I do agree Mephisto wouldn't tell Yukio no matter how polite he was (though how polite Yukio was asking might affect how polite Mephisto was answering). Chapter 94 was a complicated case since it's the wrong place and wrong time to talk about the secrets, but we can look at chapter 44 instead. That's the most far Yukio could get from Mephisto. (another example is chapter 66)
Though I have other interpretation for the other parts. Yukio (like Rin) seems never trust Mephisto. Yukio stopped playing loyal to Mephisto just after Shiro died (with that "What’s your purpose?")
Mephisto just doesn't care. He doesn't care whether Yukio is loyal or not, angry or not, trust him or not, join Illuminat or not. Everytime his answer has been consistent : I’ll hold the info when I think it isn't the time. It’s unclear what the condition is, but IMO Mephisto never expresses it like that he will never tell Yukio, but more like something is lacking so it's not the time yet.
On that note, the only reason Rin is being polite with Mephisto now is because Rin is coming into this conversation with a cool head. He has realized that he can't afford to screw this up if he's to have any hope of reaching Yukio.
I think Rin is calm and polite because he is determined, and there is no reason to be angry or fear at this point.
(Actually I do have a feeling that both Rin and Yukio aware that Mephisto will tell the secrets once Rin is determined to ask. That’s why Yukio was so upset that Rin didn’t want to learn their birth, and why Rin told Yukio he’s going to have Mephisto tell him the truth. It will be irony if the main reason Mephisto doesn’t tell Yukio is because Rin doesn’t want to know yet)
