Those 13 lives are being imprisoned in the temple tho. Even if all of her lives combined to her age, she still unfamiliar with outside world. So it's make sense she would have a child mindset.
it’s realistic though? she lived 13 lives, yes, but she wasn’t able to experience the outside world. She didn’t got the chance to experience a normal life either. She was literally abused for years and got traumatized. If it were me, I’ll go crazy fr.
You can count like she has 27 or 28 years ( 1rst life 12 or 13 years then 13 × 1 years + 2 years for 15th life). 1rst life was misery then when she arrived in the temple bad treatment once Ravienne had her on her hand torture then death. You can even say she was brainwashed by Ravienne. It's even great she was able to resist in her 15th life ( last life)...
It’s a fantasy manga… does it really matter how realistic they act as long as it’s interesting?? sure she’s 12 but she’s lived 13 lives ofc she’s gonna be more mature for a kid… and she’s finally getting love and feeling safe to act like the kid she is/never got to be, which is also fine, just enjoy the story:3