Are you sure it didn't say: "Je suis devenu pianiste"? The word pianiste can be m/f. Of course "une pianiste" must be feminine, but un/une is not required in that sentence.
In Le Passé Composé, most verbs use avoir as the auxiliary verb but a fixed number of verbs use être instead.
They are almost all verbs to do with movement (See Conjugate coming and going verbs (+ être) in Le Passé Composé (conversational past)), but also those to do with states of being (birth, death, becoming, staying).
Have a look at the following examples:
Je suis né en avril.
I was born in April.
Marie, tu es restée avec Lucas hier.
Marie, you stayed with Lucas yesterday.
Il est mort l'année dernière.
He died last year.
Nous sommes devenus bons amis.
We became good friends.
The pattern for these verbs in Le Passé Composé:
être (le Présent) + past participle of the verb
ATTENTION:
Because they're using être as auxiliary verb, their past participles have to agree with the subject of the verb.
List of the "being and state" verbs and their past participles
Learn more about these related French grammar topics
Examples and resources
Vous êtes restées dans le train.
You stayed on the train.
Il est mort l'année dernière.
He died last year.
Ils sont décédés le mois dernier.
They died last month.

Marie, tu es restée avec Lucas hier.
Marie, you stayed with Lucas yesterday.
Je suis resté à la maison.
I stayed at home.
Vous êtes restés mes petits garçons.
You remained my little boys.
Nous sommes devenus bons amis.
We became good friends.
Elles sont devenues danseuses.
They became dancers.
Je suis né en avril.
I was born in April.
Q&A Forum 5 questions, 13 answers
In the quiz Je suis devenu une pianiste why isn’t an “e” added to devenu because the person is feminine?
Yes you are correct it did say je suis devenu pianiste. I had thought that meant feminine. Thanks for the clarification Krissa
In the quiz Je suis devenu une pianiste why isn’t an “e” added to devenu because the person is feminine?
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Participle form for "vous", and for "elles"


Participle form for "vous", and for "elles"
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what about using mort or né as an adjective?

what about using mort or né as an adjective?
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Décéder & Devenir

Bonjour Chris !
All the (+être) verbs are covered in our system in two lessons: this one (for 'rogue' verbs) and the "coming and going verbs" lesson (see https://progress.lawlessfrench.com/revision/grammar/conjugate-coming-and-going-verbs-etre-in-le-passe-compose-conversational-past).
Also remember that all reflexive verbs also take être in Le Passé Composé: https://progress.lawlessfrench.com/revision/grammar/conjugate-reflexive-verbs-in-the-passe-compose-conversational-past
À bientôt !
Décéder & Devenir
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Naître in the present and past tenses


Naître in the present and past tenses
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