SAMAEL & LUCIFER - THE GRAND CHESSGAME
One cannot deny that Lucifer and Mephisto's relationship is one of the driving plots of the entire story so far. We got clued in of this as soon as the Shimane arc reared its head, introducing Lucifer's character--and that only got cemented further as the details that led up to Rin and Yukio's birth came to light.
Even at this point, however, we've only captured a small glimpse of the complex relationships between demons--and how exactly they stand apart from humans.
Given this context, it is fascinating to explore the nature of the relationship between Lucifer and Mephisto. As the two most powerful beings from Gehenna (baring Satan), they regard each other with mutual consideration and respect, even as they stand in opposing sides regarding the future of the world and humanity itself.
Although the last time they stood in front of each other in Assiah, sixteen years ago, Mephisto was complicit in the destruction of Lucifer's last vessel, Lucifer still makes an effort to extend Mephisto an offer to join his organization and its grand ambition.
But he is immediately turned down. Mephisto makes clear how far apart their stances remain and not only does he stand his ground, but we also get a glimpse of a side that he otherwise rarely reveals. A flash of genuine emotion, rooted in contempt. We will see it again shortly after the end of the arc.
From a character who is generally all sly smiles and wise cracks, it is eye-opening to finally get a peek of what was hiding beneath one of his many obfuscating layers. It's enough to give anyone pause and also pushes one to consider the fact that this is indeed the King of Time, a being who has lived through several timelines and who is not just playing a careful role as puppeteer, but also living through those experiences and the consequences of his failures.
Given that Lucifer's end goal involves destroying what Mephisto considers his favorite toy box, it is natural for frustration to breed if Mephisto has yet to find a winning path. At the very least, in the current storyline, although we can still see Lucifer still has the upper hand, it's impossible not to note the various ripples Mephisto has been careful to cultivate to ultimately come out on top (even though this has yet to come to fruition).
The game they play is a match of intellect, determination, and ego itself. The other players involved, at the end of the day, are often merely pawns; in particularly the humans.
But concurrent of the ongoing game, one must also return to Mephisto's words regarding the complex nature of a demon's relationships, for as much as Mephisto wants to win the game between them, he too is a Demon King. Thus despite looking down on the path that Lucifer's madness has led him too, we often see that he is not unsympathetic towards his situation.
That is further emphasized by the way he takes care to always introduce the descent of Lucifer's madness in a way that clearly lays out his circumstances rather than simply villainizing him. Twice we see the care he puts behind the explanation. First, with Lewin and Ryuuji, in clear explicit words, and again with Rin by showing him firsthand the reality they experienced.
To say that he only feels contempt for Lucifer would be inaccurate.
And in kind, we can say the same of Lucifer, for although he must certainly feel a level of contempt in return, rooted in the envy he feels towards Mephisto's powers and his ability to maintain a vessel, the respect he holds for him is just as presently seen in his words. Mephisto, who ranks lower to him in power, who for some years offered him hope by opening up Section 13... and who despite having become his adversary, is someone he considers strong in ways that Lucifer cannot imagine matching up to.