Learn Russian: Враг часто притворяется союзником.

The verb притворяться (to pretend) always requires the role or identity you are pretending to have to be in the Instrumental case. This is a common pattern in Russian for states or manifestations, where taking on a temporary role or appearance uses the Instrumental.

In English we say 'pretends to be an ally'. Why isn't the verb быть (to be) used here?

This is a direct translation trap! English uses the infinitive 'to be', but Russian attaches the assumed role directly to the verb притворяться using the Instrumental case. Adding быть (like притворяется быть союзником) is a common learner mistake and sounds highly unnatural in Russian.

We are using the imperfective притворяется here. What is its perfective counterpart?

The perfective pair is притвориться. Since the sentence includes the word часто (often), it describes a habitual, recurring action, which is exactly why the imperfective притворяется is required here. If it happened just once, you would use притворился.

Does the -ся on притворяется make this a literal reflexive action (pretending to oneself)?