I am a student and, along with a friend, we plan to become roommates. I know he is very extravagant. If he doesn’t pay his share of the rent, will I be held responsible for his part?

It depends. By default, the law does not provide for solidarity between roommates. This means each person is only responsible for paying their share of the rent. For the landlord to be able to turn to you in case your roommate fails to pay, the lease must explicitly include a clause of joint liability (Article 1310 of the Civil Code).