I often think about how Rei said the marriage was her choice and she had other options and how Touya said that grandma was poor and how Shouto said Enji bought himself a wife or something I don't remember
I frankly feel like it all leads to the point that there probably was some struggle for her parents (either real or imaginary) and that Rei might've been looking for ways to solve it. Now. It is very noble of her to try to help her family, but as I am, I can't think of her parents as loving and worthy of such dedication. Like, we saw Rei call her. Her mom KNEW how her daughter's marriage was turning out and did nothing? Were they living off Enji's paycheck or what???? How was Rei in the mental hospital all that time? A good parent would come RUNNING and probably wouldn't agree to any money if they meant her daughter had to marry a stranger in the first place
Either way. What would make Rei want to help her parents so much that she agreed to a quirk marriage? Were the circumstances that dire? I have trouble believing that (they don't look like they living below poverty line but that is mostly my speculation). I feel like perhaps it is something about the way she was raised that made her feel like she (a very young person at the time, I doubt she was older than 24) had to resolve her parents' financial problems
Maybe she was raised in absolute respect towards her elders. Maybe she felt unneeded, and she had the urge to make use of herself. Maybe her mother was constantly complaining to her about adult problems until Rei felt like it was her responsibility to resolve them. Maybe all of that at once or maybe something else completely