Sphere Fusion ( Composition ) allows players to level up their original spheres by fusing their replica spheres and unwanted original spheres. Original Spheres of all rarities can be leveled up, but the level cap will be different depending on the rarity.
When a Replica Sphere is used for fusion, the following fixed amount of EXP will be added depending on the rarity (Rep.F~Rep.A)
When the origianl Sphere is used for the fusion, the EXP will be 50% of EXP that fused sphere has
So if you fuse the origianl Sphere with lv.1, the EXP is 0.