Chapter 7 of The Scarlet Letter,titled "The Governor's Hall" is mainly about Hester and Pearl as they visit the governor's mansion. Hester is there to deliver some gloves that she made, as well as to inquire about the rumor that Pearl was to be taken away from her given that she is an illegitimate child.
It is also in this chapter that Pearl's behavior is described as "elfish", especially in the moment when Pearl and Hester see their reflection against the concave mirror, which made Hester's scarlet letter look huge and disproportionate. This made Hester upset, but seemed to provoke in Pearl a certain sense of satisfaction in that her mother was uncomfortable.
Pearl pointed upward, also, at a similar picture in the head-piece; smiling at her mother, with the elfish intelligence that was so familiar an expression on her small physiognomy.
Therefore, her behavior is certainly categorized as "elfish"due to the sinister way that she has of making her mother feel uneasy.