The main protagonist and the only male "D" in the world. He was a soldier in the NIFL as a 2nd Lieutenant before being sent to Midgar. Upon arriving in Midgar he is reunited with his younger sister Mitsuki after being separated three years ago and enrolled at Midgar. His dark matter fictional weapon is a gun.

Mitsuki isn't his biological sister.
To use Yggdrasil, the dragon of creation's, power Yuu has to sacrifice his memories. This is one of his secrets which he only tells Iris during the last episode of the anime.
(Source: Wikipedia)