Guys it's 2 am here and I thought of something weird and dark.
So you know how when Toudou was fighting Juuzou they said that when two demons of the same type (fire in this case) fight, the stronger one absorbs the weaker one's attacks?
What if it works that way with Rin and Satan?
What if part of Mephisto's plan is to take this little chunk of Satan's fire that's stuck in a human body, raise it to like Assiah, and then send it back to fight Satan and either 1) win or 2) be absorbed and influence Satan to at least tolerate Assiah? He'd get something he wants (ability to hang out in a non-destroyed Assiah) no matter how the final battle went.
...yeah I should probably go to sleep.
Hey, being deprived of some sleep often brought me to realize things I couldn't even notice while fully awake. I think it's something like being able to give your best shot when you're the most tired. Anyway, you had me think about something else.
Yukio was weak in the womb so the "power of the blue flame" rejected him (more or less it's how Shiro explained it). Now that you said that, I thought: what if Yukio was conceived -with- the power but since HE was weak and Rin was strong, Rin ended up absorbing his flame too? (well, after all, inside the womb they were practically one next to the other. Skin-to-skin contact) That's why they could establish a connection and Rin was able to hear Yukio calling him back at the BEI and/or this could explain that weird sentence of Yukio (which to me it's still one big mystery) "Give me your flame". Like, if Yukio had the power in him too when he was in the womb but then "lost" it, he was prolly left with this "unconscious" instinct to "call" the flame back at him.
Don't mind me, I lost 4 hours of sleep waiting for chapter 94 and now I'm worn out and talking nonesense. Gee, the wait...
Edit: to further add, I thought this theory (Yukio/Rin who were conceived with the same amount of flame power but Rin absorbed Yukio's cause he was too weak in the womb) could explain a couple things. What if Yukio's constant desiring to be "stronger" (than Rin) it's nothing else but another unconscious instinct he got from having being "stolen" of his share of power? Like, he feels something's missing inside of him and can't define it other than willing to be stronger than Rin but, in reality, he's craving his missing flame?
Also, I think this could help explain Lucifer's words too. The "secret" behind Yukio's birth could be: Yukio HAD the power too but Rin absorbed it. So, "learning the truth" could help Yukio as in: if Yukio knows that Rin's in possess of his flame, he will know what he'll have to do: get the flame back from Rin. And the "potential" could be the ability to "regain" back his flame. Potential which could be unlocked ONLY by knowing the truth. It makes sence imo.