Tsubaki was a young miko (Shrine Maiden) in training like Kikyo when she was alive. She studied under a priest at a Shinto shrine as an apprentice and she and Kikyou were rivals, sometimes battling side-by-side to defeat youkai, Tsubaki always attempting to better Kikyou using her own holy power. While Kikyo and Kaede were traveling the countryside to visit small villages and help slay youkai, Tsubaki laid a curse on Kikyo, telling her that as a miko she must abandon her heart, for if she ever fell in love with a man she would lose her holy powers and suffer an untimely death answered by Kikyo's cool denial.
Tsubaki only places the curse on Kikyo in the anime.
Hanyou (half-demons) are a mix between human and yokai (demons). Some are born from a parent of each race, whose love superseded the norms set by human and yokai societies. Others, however, are born through unnatural and malicious means. Read all about the hanyou in this article.