Compi is a cute, colloquial slang word for "buddy" or "pal" in Spanish. It's a shortened, more informal version of the word compañero (companion).
In the song, Alvaro Soler promises, "ser tu compi en cielo azul y en tempestad" (to be your buddy in blue skies and in storms). Using this word instead of a more formal one like amigo or compañero adds a layer of warmth and deep friendship to the romantic promise, making it extra special.
Spanish singer Alvaro Soler kicks this track off by marveling at the sheer odds of true love: Entre ocho mil millones no sé cómo te encontré ("Out of eight billion people, I don't know how I found you"). He lands on her personal world like a space traveler without luggage (Yo venía sin maleta y sin pensarlo me quedé – "I came without a suitcase and stayed without thinking"), leaving behind a dull life lived in black-and-white to finally see color. What makes this romantic confession so charming is that it skips the grandiose drama and goes straight for ordinary mornings. Alvaro adores her natural self, singing that while she looks great dressed up, estás más guapa recién levantada ("you're prettier just out of bed"). His ideal vision of domestic bliss is eating breakfast kisses in bed (desayunando besos en la cama) and making a playful rule: si peleamos que sea con almohada ("if we fight, let it be with pillows").
Beyond the cozy daydreaming, he makes a real everyday commitment. He promises: Yo prometo ser tu compi en cielo azul y en tempestad ("I promise to be your buddy in blue skies and in storms"). Notice his choice of the casual Spanish slang compi (short for compañero, meaning "pal" or "partner in crime"); he wants an equal teammate through thick and thin, someone to learn from while they grow together. By the time he asks the night sky to apaguen la luna ("turn off the moon") so they can stay in the dark, he is completely sold. It is warm, silly, and honest about wanting to share regular, unglamorous days with your favorite person.