My theory on mordret's flaw

Just like Sunny is a shadow, mordret is a reflection. So what if the minute he got his powers the real mordret the one who was a normal kid, had this reverse (his mirrored self) take In charge of his body and the real personality is in a mirror, like a split personality thing.

Mordret said he didn't bring his flaw with him in 3rd nightmare, probably because his real self in the mirror is his vulnerability.