confused

yes, he was reborn as a sin for saving Rix from death. so it is Nuah's body but he has none of their memories (rix's & nuah's)

the original nuah died/ disappeared cuz he went against his god, he kinda sacrificed himself for rix (i don't really remember)
he was reborn and lost all his memories, so it's not the same person anymore, he just has the same appearance as the old nuah that rix knew

so how come rix remembers the old nuah

he was supposed to forget too but banam (the devine beast) spared him

well the author has been hinting as if the original Nuah isn't completely gone. A couple episodes back the gold lizard divine beast also made it seem like he's not gone too. I'm guessing maybe the old Nuah is locked within the new Nuah and once a certain condition is met he'll remember. I've seen other comments also suggesting this

misread youre comment, i thought you asked why the new nuah remembers the old nuah. Rix asked the gold lizard divine beast to let him keep his memories, if I remember correctly he did this as a sense of punishment for himself b/c he felt guilty of what Nuah did. especially since he kept ignoring old Nuah's affection b/c he was scared he would eventually leave & neverfull told him his true feelings

he is basically the same person he just lost all his memories regarding rix, making him a "different person" which is way too dramatic in my opinion just make new memories lol