He clearly has Kuro's tail in this photo, and what looks to be the hands of a werewolf. SO, what if their Sensei gave parts of himself to the vampires he has created?

Sleepy Ash of Sloth: The tail of a lion

Old Child of Pride: The ears of a bat

Doubt Doubt of Envy: The scales of a snake

The Mother of Wrath: The claws of a wolf

Lawless of Greed: The spines of a hedgehog

World End of Glutton: The tusks of a boar

All of Love of Lust: The wings of a butterfly

And Who is Coming, Melancholy: The fur of a fox

But perhaps the only Servamp who knows this is Sleepy Ash. The only one who was greeted with the true form of the creator. The only one who knows that the creator is truly a monster. Pride and the other Servamps clearly believed that he was human, that he was mortal, but Sleepy Ash clearly knew something the others didn't, and this was it. The reason why he had to kill him was to keep his siblings safe. Of they saw what created them, it would have caused chaos with the vampires. The face they knew would have been shattered.

The kind expression they may have gotten would have been gone. Now for the ones who wanted to kill him, its possible that he did some seriously fucked up shit with them, especially All of Love. After all, kind, sweet smiles like this can't be entirely trusted if you don't know the person at all. Same for Pride and Envy, they could have witnessed a side to him that's more demeaning to his true self than compared with Wrath, Greed, and Gluttony.

So with Sleepy Ash being the oldest Servamp, who knows what he's been through to be changed into one. We don't know the cause of his friend wanting him to be one as well. Who knows if he was sickly, or what he went through to become what he is.

So I HC that he's actually very sickly. With what? Who knows. I believe it could be something with his lungs, or maybe stomach. Something that slows him down enough to deem him 'lazy'. With him being the first Servamp, who knows if their creator messed something up? Maybe he's got some type of internal damage?

However, with all of this. He has to live with it permanently. He's immortal now. It's there, but he can no longer die by whatever he's sick with. So he has random busts of illnesses that would seem highly alarming to a human.