
Major spoiler for the main story:
Read at your own risk---
It's not that his arrogance is toned down. This is Clayman's original character. This is why Laplace in the canon questions Clayman's plans and behavior. As it turns out, Yuki has a skill that allows him to manipulate his target's thoughts and emotions. Yuki slowly amplified Clayman's arrogance, inferiority complex, and fear of being too weak to help his friends. Over the decades, Yuki carefully cultivated the worst version of Clayman to turn him into a puppet.

Here's your spoiler alert:
Major spoiler from the web novel,
You've been warned.
Laplace figures out what Yuki did way too late. Yuki also used his skill on Curse Lord Kazalim, the boss of the Moderate Harlequin Alliance. Kazalim duels Leon to the death but has a moment of clarity before he dies. He asks Laplace if this was what he (Kazalim) wanted. That was when the pieces fell into place for Laplace. Laplace's anger toward Leon had clouded his mind. Laplace wanted revenge. He forgot that the boss he knew would not have gone so far to get revenge. This was not what Kazalim would want. Laplace realized that Kazalim and Clayman started acting out of character after they met Yuki. However, it was too little too late. Kazalim was dead, Yuki was too powerful, and the Moderate Harlequin Alliance had outlived their usefulness. Yuki tried to eliminate them as soon as Kazalim died. Laplace dies trying to protect Footman and Tear. It's been a while since I read the novel, but I think Laplace fails to harm Yuki. Footman and Tear end up working for our friendly neighborhood slime.

It is similar, but Yuki can do more with his skill. Now the light novel I read was a streamlined version of the story. It lacks a lot of details like Charybdis and Maribel. However, I'll make an educated guess that Yuki is pulling a Milim here. Maribel's skill is unique, but Yuki's skill is on par with Raphael. Since his skill trumps hers, I think he's pretending to be under her control. Her power influences the desires of others. Yuki's ability is more complete. He prefers to subtly influence others and brainwash them like he did with Clayman, but he can puppeteer people against their will. They are aware of what he makes them do, but they can't stop themselves until they complete whatever he ordered them to do. Their desires do not matter. In the version I read, he can completely dominate a person and give them three orders that they cannot disobey. He can't control them again after this so he has to be very careful with his commands.
WAAHHHHHH I want him to be the main character so bad...Seriously what a great character development...If only this is canon then I will finally die happily, BUT it's not Anyways the art is good and with his arrogance and pride being toned down, his actions became suddenly cuute in my eyes....