Heroine is a play on words, a homophone. He is saying “let’s hear it for our heroine” as in the girl mentioned in the verse. But he is also using it as heroin, a highly addictive drug. He is trying to get across that his love for this girl is addictive. Heroin addicts become so dependent on their drug that it becomes the only thing they live for; he’s saying that this girl is his reason for living.
To help improve the meaning of these lyrics, visit "Don't Look Down" by Kanye West (Ft. Big Sean, Lupe Fiasco & Yasiin Bey) Lyrics and leave a comment on the lyrics box