Shinju (神樹 Divine Tree) is the god in the universe of Yuki Yuna is a Hero that remains in Jukai. It is stated it chooses girls with high aptitude to fight Vertex. People mainly refer to it as "Shinju-sama" showing their high respect for it as it protects humanity.
The Shinju-sama hasn't made much of an appearance yet. The literal meaning of it means Divine Tree and the outlines in the anime make a tree so it is most likely presumed it is a tree.