Hybrids are a combination of two species, often between a Human and Demon, a perfect Hybrid is hard to come by and so far has only been achieved through the usage of the Chalice of Judas Iscariot. Rin and Yukio Okumura are naturally born Hybrid twins, but are considered imperfect and unstable, while upon touching the Chalice however Skoll's former Celestial nature allows him to become the perfect and original Demon-Human Hybrid.