In late 2024 I wanted to draw each of the seven demons of sin to showcase each of them individually

This is leviathan, the demon of envy
She is Leonard's youngest.

She's actually the adaptive cuckoo daughter of Leonard, as she is biologically the daughter of Lucifer (who was god of Hell at the time) and Leonard's ex-wife, the Lilith. In fact, it was this cucking that drove Leonard to kill Lucifer. And even though they tried to make it work, it's also what ultimately led to Leonard's and the Lilith's divorce.

Leviathan is aware of all that, hence why she is the perfect choice for the demon of envy. She's also a year younger than Kno, this and has a huge rivalry with her. Almost sibling like, even though they are cousins