Yu☆Gi☆Oh!, Nurarihyon no Mago Recommendations
in this shonen anime there are no duel monsters but there are yukai and the hero has two personalities just like yugi while the heroine reminded me alot of anzu..thank god there is no seto kaiba replacement here!!
The main similarities I found were:
MC is innocent and childlike.
Transforms into a different version of themselves that makes them stronger.
People don't find out about either transformation until later on down in both shows