Alternatively, I could avoid something that goes for "faster rate of learning," and go with one of the following ideas I just thought of while mulling over my character's setup.

World of Disguise - Some sort of skill that allows for limited "shifting" for aiding in disguises. Could be limited to usage of "once per long rest" or something, forcing careful usage of the skill. Could also be limited to whoever she's observed enough to know their general body structure.

Vocal Mimicry - After hearing a person's voice a certain number of times, Glaive can mimick their vocal patterns, assisting in disguises.

Had a third idea but somewhere between writing the prior two and having some soda, forgot it.