Santana Lopez is a fellow Slytherin and is in the year above him. Theo has always liked Santana her "bitchy" attitude and the two of the took a liking to each other when they first started talking to each other on Theo's first night in the Slytherin Dungeon. Santana told him everything he needed to know about his House and was the first person in the school that Theo really trusted and bonded with.
Santana was named the "Deputy Snake" by Theo and Jackson and the Slytherins will have to answer to her if both Theo and Jackson have other duties to attend to and are therefore unavailable. Santana loves to tease her rival Alex with the fact that she got a high position within their House while she's still an "ordinary" Slytherin.