Du'a for good character

اللَّهُمَّ كَمَا حَسَّنْتَ خَلْقِي فَحَسِّنْ خُلُقِي

Allahumma kama hassanta khalqi fahassin khuluqi

O Allah, You have beautified my body, so do beautify my character.

Musnad Aḥmad 3823. Saying it on looking in a mirror is reported only with weak chains.