It's Amethyst. Or, more specifically Amethyst Kelly. If you're wondering how she came about Iggy Azalea - she went down the highly original pornstar moniker route of teaming her first pet's name and street name.
I dunno, maybe she's nervous, but this is VERY flat - and that's not just a reference to the vocals (Corden's rap skillz never fail to impress). It's just a bit... meh. Super muted. Especially when compared to previous participants. At least James drives her to a bridal shop ahead of her wedding to Nick Young.