Posing as someone else typically uses Deception checks.
In the situation you describe, the PC is posing as a bodyguard and doesn't want to be exposed. Whether this means maintaining a disguise, or hiding her motives, or keeping up a convincing guard persona, the Charisma (Deception) skill is probably most appropriate.
Deception. Your Charisma (Deception) check determines whether you can convincingly hide the truth, either verbally or through your actions. This Deception can encompass everything from misleading others through ambiguity to telling outright lies. Typical situations include trying to fast- talk a guard, con a merchant, earn money through gambling, pass yourself off in a disguise, dull someone’s suspicions with false assurances, or maintain a straight face while telling a blatant lie.
To determine the PC's success at posing as a guard while the NPC is observing her, you would probably have the PC and NPC roll contested skill checks. Typically the NPC would roll a Wisdom (Insight) check versus the PC's Charisma (Deception) check, although as the DM you have the liberty to choose which skill the NPC rolls, depending on the circumstances.