The boots are the 3rd thing Nella calls out while transforming, following the shield.
Given that these are armor, the technical term for the components of her worn metal leg armor (not actually spoken in the show) would be:
- poleyn or genouillere for the portion protecting her knees
- greave or jambeau for the portion protecting her shins
- sabaton or sollerert for the portion protecting her feet
Nella does not appear to wear plate armor protecting her thighs (called a cuisses or tasset) instead wearing chainmail (called chausses) on the proximal halves of her legs, much like her arms above her gloves.
Nella also has protection for her lower body in the form of a pinkish-purple skirt which has not actually been called out by name in the show. Although this might just be decorative, there is a form of armored skirt called a "bases". Underneath this it's also possible that her arm chainmail could be part of a larger chainmail shirt called a hauberk.