Heartwarming / Obey Me! – One Master to Rule Them All!

• When Leviathan gets angry during the Tales of the Seven Lords competition, you have the option of calling out to Mammon for help. Should you choose that instead of running away or closing your eyes, Mammon will try to run towards you in an attempt to protect you from his brother.
• In a dark way, the fact that Lucifer's siblings chose to side with him when he rebelled against the Celestial Realm. While the present state of their relationship seems chaotic and unhealthy, their relationship with their eldest brother back then was strong enough for them to choose to fall from grace with him.
• In Lesson 21, when you and Solomon return to the Devildom on short notice. You drop in (literally, in Satan's case) on a student council meeting and all the brothers, with Diavolo and Barbatos included, are overjoyed to see you again and immediately invite you to stay at the House of Lamentation.
• The "One Too Many Insults" side story has a very sweet moment in the final two parts. When Mammon runs away from home, his brothers initially believe he's being overdramatic like always. However, they start getting concerned after a few days pass by. Just as Mammon stops by to get his phone charger, he overhears his brothers voicing their regrets for teasing him too much and wanting to look for him. Touched by their words, he tries to sneak back out only to cause some noise and getting their attention. His brothers rush out to see who it is and they're glad to see Mammon is back instead of insulting him or making any usual accusations of him trying to steal money.
• After you and Beel return to the House of Lamentation in Chapter 14, Lucifer leads you and the others to the lounge to finally come clean about Belphegor and the reasons why he locked up his brother, as well as why Diavolo and Barbatos took him away. During the awkward silence that ensues, if you pick the option “Would it be better if I weren’t here?” each brother in turn objects to the offer outright. Their reasoning is that this involves you as much as them, since they see you as family now.
• This post on Mammon's Hidden Depths.
• When your birthday comes around, the game celebrates it in style. You're treated with a short "Happy Birthday!" video, and periodically you get calls and messages from the demon brothers wishing you a happy birthday. To describe one message, Asmodeus waited at the crack of midnight just so he can be the first one to send his birthday message to you.
• During a chatlog, Lucifer demands to know who broke a painting and Mammon confesses, to the shock of his brothers. Lucifer surprisingly chooses not to punish him for it. Later, Simeon messages Lucifer asking about what happened and Lucifer reveals Belphie's the one who broke the painting when he bumped into it when he was half-asleep, so Mammon took the blame. Lucifer knew this, which was why he chose to let Mammon off the hook, commenting that Mammon protects his little brothers. The So Proud of You is unspoken but very clear.
• In the reaper cave segment, when the brothers finally reach the final level of the cave, they notice Beel's soul candle is nearly burnt down to the end and the flame is in danger of going out, meaning Beel will die very shortly if they don't fix it. Lucifer immediately steps up and pours some of his candle's wax into Beel's. When Beel points out that means Lucifer's lifespan will shorten too, Lucifer says he doesn't care about that as long as it means they can buy some more time for Beel. This prompts the other brothers and MC to also pour in some wax, which is what breaks the reaper's curse.
• In "The Rulebook" sidestory, an argument between Lucifer and Mammon has Lucifer recall a memory from when they all Fell and became demons. In the memory, implied to be shortly after the rebellion, Mammon informs Lucifer that the rest of their brothers are resting and their injuries will heal in time, though the twins are still reeling from Lilith's death. When Lucifer expresses regret and guilt for dragging his family down with him, Mammon insists that they all chose to rebel with him and they don't regret it. As their older brother and the Avatar of Pride, Mammon tells Lucifer that he needs to keep his head held high and be confident to make everyone around him feel just as confident. In the present, Lucifer thanks Mammon for making him the demon that he is today (although, he's also saying this as he's stringing Mammon upside down).