Thinking about these two again...
1. Thinking about how their motto is the same, but just the way they talk about it.
Diavolo is so determined and doesn't hesitate, giving us an answer immediately. But Lucifer? He seems like he didn't expect that question at all and he never had ready answer for it. He gives himself some time - he's repeating question, "maybe".
You can't convince me otherwise: originally this motto belonged only to Diavolo.
I like to think that Lucifer is...looking up to Diavolo, but in his own way. I can see how Diavolo’s determination, reliability, and openness would only further influence Lucifer’s attitude towards him, and could have become a solid foundation for Lucifer to finally be able to trust someone again, especially after The Fall. Lucifer is loyal to him because he's confident in Diavolo’s dream, his principles and his words. And, most likely, after thousands of years of friendship with Diavolo, Lucifer also unconsciously began to instill new, similar to Diavolo’s, habit in himself, began to look at things differently.
2. Thinking how their voices become softer when they talk about each other in Main Screen Dialogues.
The fact that Diavolo just smiles talking about Licifer, but Lucifer looks happy talking about Diavolo. Which is quite unusual to him: he openly shows us his true feelings, he even talks about the days before the Fall. He's happy to have known Diavolo, and for SO long.
"It feels like I've known him forever" from Diavolo still shows us just how much he actually cherishes Lucifer, too. He's so imbued with this particular black-haired proud demon, that he doesn't even remember (or doesn't want to remember) the times when this demon wasn't next to him, because for him it's something strange, something unusual, because hey, what do you mean he hasn't been next to me since the beginning of time?
3. Thinking how their magic is similar at some point.
Diavolo can see memories with physical contact, Lucifer can read memories.
Only they two are able to present healing magic.
I wouldn't be surprised if it turned out that, deep in the past, Diavolo was teaching Lucifer some complex magic/curses/hexes and such.
4. Thinking how actually Lucifer is down bad for Diavolo just as much as Diavolo is for him.
Yet it's not that obvious, it's hidden in his words and actions, because it's Lucifer, and everything that includes him is always hard to notice.
Lucifer does not regret that he swore his loyalty to Diavolo. His desire to see the world the way Diavolo wants it to be is absolutely sincere. He's very devoted to Diavolo, and this devotion is even ridiculed several times by his own brothers. This devotion is so strong that it makes him toss and turn between Diavolo and his brothers for the entire OG Season 1. He didn't like when in OG Season 2 Diavolo chose Solomon instead of him, to the point that Lucifer constantly complaining about it to MC.
Lucifer hopes that Diavolo will continue to need him.
He dislikes the idea of Diavolo doubting his abilities so much that he tries to cook Diavolo’s favorite cookies and ends up failing it anyway. He doesn't mind the time loop in OG Season 2 only because it means he will be having tea party with Diavolo all over again.
He trusts Diavolo so much that he's allowing him to keep his photos (and keep making new ones).
He remembers every little detail about Diavolo just like Diavolo remembers about him (Lucifer literally remembered the music Diavolo liked, found music sheet and played it on piano for Diavolo on his b'day).
He recognized Diavolo’s feet just by photo.
He never allows anyone to drop "Lord" when addressing to Diavolo, even his brothers.
He was ready to spend an eternity with Diavolo if Diavolo was going to absorb Dark Santa's magic in one of the events.
He's allowing himself to be playful/frivolous with Diavolo. He wears idiotic hat for photo with Diavolo, he plays chess with Diavolo in helmet, he plays Mario Kart with Diavolo, he suggests "whoever empties the bottle of Demonus first wins", he's very excited when he's getting a chance to play a game of chance (casino) with Diavolo.
His favorite clocks is the one Diavolo gifted him. He treasured the ship in the bottle that Diavolo game him so much that he put it in a place where he was able to see it all the time, in the living room.
"What Diavolo wants, I want" direct Lucifer’s quote.
He sees how hard Diavolo tries to get to know his brothers better, and Lucifer lets him to move closer to his family. He's happy for Diavolo that he finally is able to talk freely with his brothers.
He knew and he saw just how much Diavolo have done for them all after The Fall, that he felt ashamed to ask for something more.
He understands Diavolo from half of word, he asks Diavolo for advices, he always yapping about his brothers to Diavolo.
He comforts Diavolo when he sees that Diavolo is upset in one of NB hard lessons.
Do I ever need to continue..? 😔
