Envy

A snake. No really -- she's a naga, so literally a snake. 

She's a poor girl who had died full of regrets in her life, thus making her a perfect candidate for the position of Envy. She has a crush on Lust- which wouldn't be so bad if she wasn't a Yandere. Yep. She's the one with the extremely high kill count from the amount of times she's gotten jealous over the flings Lust would go after. She loves attention, but does get shy about it. 

She sees herself as somewhat of a mother figure to Sloth.

It's Wrath and Gluttony that keeps her in check to make sure she doesn't go overboard and kill too many people.