User blog comment:B214/Light Novel Volume 16 spoilers/@comment-75.93.25.66-20140105180833/@comment-23847729-20140115035947

And Just to add to what  B214's wrote, Valerie have a Longinus Sacred Gear.

Like in chess, each Each Pieces have a certain number of values. King: Unknown, Queen: 9, Rook: 5, Knight and Bishop: 3, and Pawn: 1. to make a person or creature into a Devil, depending on a person's abilities depends on the number of Evil Pieces to use.

Since Valerie have a Longinus Sacred Gear, it would take a lot more pawn pieces in order to make her into a Devil.