"Oh. Now that's a good question" Mammon's grim widened considerably, clearly satisfied with Leviathan's initiative to ask him about who he planned to take with him for a little while - after all, it was the perfect opportunity to demonstrate the flawless justificative he made in the instant he decided to use his well deserved vacation to tease her - visibly enjoying himself as he started to make his way towards her office, conveniently ignoring the need to receive a confirmation from Lucifer.
"I actually had quite the trouble with making this decision, you see?" he nonchalantly approached her desk, making his way through the unsettling number of documents that covered the room as he approached his fellow sin.
"I mean, being the reasonable person that I am, I know pretty well that we aren't supposed to slack off when considering how much work we already have to do in a daily basis and how doing so would make this already unhealthy quantity even more alarming. That being said, I believe that overworking ourselves would be even more prejudicial to the underworld, thus I came to the conclusion we should logically take periodic vacations to avoid having to deal with such issues." He explained while taking one of the piles of documents positioned directly besides the sin and relocating it to clean a spot for him to sit in the desk before progressing with his not-so-convincing speech.
"With that in mind, I started considering who I should take to test the verisimilitude of my conclusion. To start with, I naturally started to observate the sins to conclude which would take the most benefit from such an opportunity, and after a some serious investigation I concluded that... We actually don't have that many sins in need of a vacation." He dramatically paused his speech to glance at Leviathan, as if to confirm that she was still paying attention to his bullshit.
"I mean, amongst the sins Asmodeus, Beelzebub and Belphegor are already relaxed to the point where their working doesn't even count as a burden" He stoped for a moment to momentanuously turn his head towards Belphegor - who conveniently entered with his newcomers in the middle of his speech - throwing a glance towards the other sin that clearly meant something along the lines of "don't you dare to spoil my amusement" before continuing
"Satan is so freaking naturally nervous that he already counts as a lost cause - and thus wouldn't be fit as a test subject - and Lucifer not only can't take a vacation - as his responsibilities as a head of state require his constant presence - but also counts with the help of a relatively competent assistant - considerably reducing his burden" He once more stoped, showing a smile that almost looked innocent as he finally voiced his conclusion:
"And finally, Leviathan - You - is always so dedicated to working in despite of not showing any particular trait that makes you immune to the burden brought by such a massive amount of work that you can only be regarded as the perfect subject to prove my theory. You're the only one yhat I can possibly take with me that can effectively make this vacation profitable for the underworld... That is, if you accept going to a date with me for the sake of our beloved realm." He finished with such an eloquence that it almost looked like he had absolute faith in what he said - a clear bullshit, as he honestly didn't believe that his crazy speech made in the spur of the moment would effectively manage to convince Envy, as he effectively concentrated more in taking some amusement from the situation and hopefully impress some of the newcomers that came with Sloth in the process. Well, he wouldn't complain if it actually worked though... A date with a cute girl isn't something he could possibly reject.