So I saw in that "Empress, Ambassador, Tyrant" fic that Noblesse actually rises higher than Diabla, and I was wondering why that was if Diabla defeats Sult and Noblesse just essentially marches into a throne room and says "fuck you all i want a very specific chair"?
“Fuck you all I want a very specific chair.” Nicely put.
As for your question….. Hooo boy, prepare for some shit analysis.
I sort of assumed that how much power Lu can regain depends largely on Ciel. Take a look at the differences between Demonio and Royal Guard.
Demonio is a passive player, fighting for Lu as he’s always done, but he only transforms because Lu essentially forces him to. Diabla’s power overflowing into Ciel is what creates Demonio, but then Demonio isn’t free to act on his own and elevate Lu further because of two reasons - Diabla keeps him under strict control (that mouthguard really does look suspiciously like a gag, there’s a win pose where he legit loses control and Diabla looks happy about it, there’s a lose pose where Diabla viciously kicks Demonio who’s visibly distraught about failing) and also he’s really too busy trying to control his own demonic energy, or rather Lu’s demonic energy, to waste his focus on anything else.
Royal Guard, on the other hand, is a very active player. He transforms first essentially out of a desire to serve Lu the best he can and make all her goals come true, especially the goal of retaking her throne. (It’s stated that his transformation into butler is “unintentional”, too, which I find interesting, but I digress.) Royal Guard acquires and controls his power deliberately in the support of a very specific goal. And it works - Noblesse is a class that regains her power only because of Ciel’s sweat, blood, and tears. What’s more, unlike Diabla, Noblesse recognizes the effort Ciel puts in for her. She gets cocky sometimes, and there’s no doubt that Ciel is technically her servant (unlike in the CH/DL path where they’re almost totally equal) but Noblesse comforts Ciel when he loses, and she brings her own skills to the table (spears and souls) and both Lu and Ciel use them just as often as they pull out Ciel’s Marbas.
So Noblesse has a stronger contract with Royal Guard than Diabla has with Demonio. I assumed that with a strong Ciel behind her, supporting her like Royal Guard’s entire mission is to do, Noblesse would have the freedom to aim higher. While Diabla’s mission was simply to regain her throne and not die along the way, Noblesse’s was to regain her powers and acquire the throne she’d been playing for when she was first betrayed. Remember, Lu essentially lost her power in the first place because her “Ciel” stand-in was untrustworthy and betrayed her. With Ciel behind her willingly and fiercely, Noblesse will get farther than Diabla will, even though Diabla arguably has more power than Noblesse.
Sorry if that didn’t answer your question. Tl;dr just in case I didn’t explain clearly enough - RG is willing to work hard for NB and does so, DN is willing to work just as hard but really doesn’t have the capacity or support from Dia to do so.