I see and love the Pride demon and see that the Sin Kingdom has that theme…all the sins lol. So I was thinking of Envy of two ways:
The first to be kind of the opposite of Pride. Where Pride you steal from them for every skill you have MORE of then the target, Envy could steal from the target for each skill that it has LESS of.
The second idea is something like you do X damage divided and you do X damage more where X is the difference between the loss between your skills. Like if you do 20 damage base…then you have 20 attack the target has 30 attack that’s 10 more added. The same with armor and health and magic, the other skills.