Learn Spanish (Latin America): El gerente no es estricto, más bien es hipócrita y manipulador. - The manager is not strict, he is rather hypocritical and manipulative.

Both can follow a negative statement, but they have different nuances. While sino completely replaces the first idea (not X, but strictly Y), más bien acts as a clarification or a better phrasing. The speaker is saying that instead of calling him strict, a more accurate description of his behavior is hypocritical.

Why does hipócrita end in an 'a' when we are talking about a masculine subject (el gerente)?

The adjective hipócrita is invariable in gender. Much like words ending in -ista (such as egoísta or optimista), it always ends in -a whether you are describing a man or a woman.

Could I place más bien somewhere else in the sentence?

Yes, más bien is quite flexible. You could place it after the verb, as in es más bien hipócrita. However, putting it at the very beginning of the contrasting clause strongly emphasizes the correction of the previous statement.

Does más bien always need a negative statement like no es estricto before it?

Not at all. While it contrasts two ideas here, you can also use más bien in a standalone affirmative sentence to mean 'rather' or 'quite'. For example, El gerente es más bien callado means 'The manager is rather quiet'.