When it refers to the zodiac sign, the plural of “géminis” is "géminis.” The adjective "géminis" is invariable, which means its form does not change according to the gender or number of the noun it describes.
La adivina me dijo que libra sería el compañero ideal para una géminis.The fortune-teller told me that a Libran would be the perfect match for a Geminian.
An adjective is a word that describes a noun (e.g., the big dog).
Creo que voy a dejar a Juan porque soy géminis, y una relación con un piscis sería un verdadero desafío.I think I'm going to break up with Juan because I'm a Gemini, and a relationship with a Pisces would be really challenging.
