Ah, my headcanon for the often asked question: What's the deal with Mamoru's money situation?
In the live-action PGSM, this was actually answered: Kusaka Hina's father.
But why Mamoru? Did he just let Hina pick him out like a puppy? Probably not. If I had to guess, Mr. Kusaka probably knew Mamoru's parents in some way and felt the need to take him in. But rather than adopt him as a child, he wanted Mamoru to marry into his family, which was probably seen as more socially acceptable. He and Hina had their hearts in the right place, it's just that forces much larger than them were at work.
But to further that, in my headcanon it's always Mr. Kusaka. In every canon. He just happened to have a daughter in PGSM.
Bear with me while I set up the entire backstory from Terran Moon that this stems from...
In Terran Moon, his name is Kusaka Toshie and was bullied by Ishiei in middle school. Hiyo, being the righteous and nosy girl she is, stepped in and demanded he cut that out. Surprisingly, he did and the three of them ended up friends.
Unfortunately for everyone involved, a love triangle develops but no one says anything for various reasons. This continues on until they're adults. Ishiei settles into an average office worker life, Hiyo and Toshie are corporate ladder climbers, rumored to be a power couple despite not dating.
It comes to a head when Hiyo receives a promotion that would allow her to work abroad, her dream career. Toshie panics, being so in love with her (as his childhood savior who brought him out of his shell) that he can't imagine her not being there. He confesses and asks her to stay with him. Distraught, she confides in Ishiei who also admits that he doesn't want her to go, but also feels like it's not his place to make her stay. Japan was too small for such a great lady like her.
Hiyo, confused about her growing feelings for Ishiei, takes the promotion and leaves without telling either of them. Ishiei and Toshie, knowing what happened, grow apart and throw themselves into their work as well. Hiyo does eventually return to Ishiei and the two marry. Toshie quietly slips away not wanting to confront either of them.
Now here's the split. For the manga and anime canon, Toshie (now a wealthy, but somewhat cold business man) hears about the accident and finds out about Mamoru. He gets his attorneys to work, setting up Mamoru's inheritance fund and assets, making sure no one can lay a hand on it (Ishiei had some shifty family). He did consider actually taking in Mamoru but it wasn't really proper, not being related to him, and also because Mamoru looked so much like both Hiyo and Ishiei it really bothered him. Instead he opts for his secretary to keep tabs on him as he grows up from a distance. It's the only way he knows how to care for him, to make sure he's financially comfortable.
Once Mamoru leaves the orphanage, Toshie's secretary greets him, saying a private patron hired her. She explains that Mamoru is already set up to attend Moto-Azabu, he has an apartment nearby, and a bank account already at his disposal. Mamoru goes along with it because, at this point, why argue? But his most cherished possession is the first thing she hands him, the family registry with his parents names on it.
For PGSM, a depressed Toshie was introduced to his wife by Hiyo before they pass away. The couple end up happy and grateful and name their daughter after her, Hina (same first character). This obligation to both Hiyo and Ishiei is why the Kusakas take in Mamoru.
So really, it's only Terran Moon he's not involved. I wouldn't say he's still bitter about it though, it's just awkward to try to rekindle that friendship under those circumstances.
If you'd like the comedy option, you can imagine Toshie's secretary is secretly following Mamoru around to report back to Mr. Kusaka... and actually knows all his secrets.
"Sir, Mamoru just donned a tuxedo and is throwing roses at monsters. I believe he's reciting poetry about... grocery stores?"
"...........just keep an eye on him."
And that was my much too long headcanon for a character that never actually appears and is barely mentioned. And honestly, he likes it best that way.
