The Kaga Kaga no Mi is a Logia-type Devil Fruit that allows the user to transform, control and create glass. The word "Kaga" is a pun from the Japanese word "Kagami" (鏡) which means "mirror".
Kaga Kaga no Mi
Kaga from the word Kagami (mirror)
Create, control, and transform into glass
The main strength of the Kaga Kaga no Mi is that the user can become a mirror themselves. When they are hit by a physical attack, the user will crack then shatter like broken glass. However due to the nature of this Devil Fruit, this only gave the user the impression that they had attacked the user when, in reality, they had attacked a mirror illusion.
Another uniqueness to this fruit is that the user can also can make multiple mirror illusions for gathering intelligence and battles.