Learn Finnish: Meidän kekseliäs ystävämme osoittautui sankariksi tässä pelissä. - Our clever friend turned out to be a hero in this game.

This is a strict rule (rection) required by the verb osoittautua (to turn out to be). Verbs that express a transformation, outcome, or what something is revealed to be generally require the translative case (-ksi) for the resulting identity or state.

If the friend turned out to be an adjective instead of a noun, would the translative rule still apply?

Yes! If you wanted to say the friend turned out to be clever, the adjective would also take the translative case: ...osoittautui kekseliääksi. The verb osoittautua governs both nouns and adjectives describing the outcome.

Is it necessary to use both the pronoun meidän and the possessive suffix -mme on ystävämme?

In standard written Finnish, the suffix -mme is mandatory, while the pronoun meidän is technically optional and adds emphasis. In casual spoken Finnish, however, the pattern flips: people usually just say meidän ystävä, keeping the pronoun but dropping the suffix entirely.

Why doesn't the adjective kekseliäs take a possessive suffix too?

While adjectives must decline to match their noun in case and number, possessive suffixes are an exception. They only attach to the head noun itself. So the phrase correctly remains kekseliäs ystävämme, and never becomes kekseliäämme ystävämme.