Very interesting concept.
I think folks in the mirrored world should be coloured differently instead of invisible, as I feel the latter would be too confusing. Also, the folks in the mirror world should probably only be able to walk so far from the mirror, like a large sphere around the mirror.
I think a cleaner way to give the character a way to break mirrors would be to make him deal more (3x for example) dmg to mirrors, that way he can reliably always break them before the enemy can.
It'd also be neat if projectiles shot at the center glass of the mirror went inside of the mirror world, that way you could use mirrors as a sudo shield of sorts, with the borders still being damagable.
Correct me if Im wrong, but the 3rd ability makes it so that when you walk into a mirror, your model still shows up on the map and thats how you can still be killed (thats how I understood it) and if that is how that works, I feel the mirror should also break as that copy dies, forcing you into the real world again. I think you can streamline it further by making it so only you can apply the Shattered image debuff, and you activate it by shattering the mirror, and if enemies shatter the mirror while a debuff is active, it heals them instead, similar to how shattering the mirror while inside MW either heals/damages you depending on who shatters it.
As for Reflection, I think its great! Rubick style ability with a completely different base kit, but I feel like it still works and fits the character, nicely done.
As a last suggestion, I feel like shattering the mirror on enemies and even perhaps the shattering image debuff should both stun, both for more utility and because I feel like theyd need it, as they dont have much defense if they get jumped