To tell you the truth, I think Mario is Masako's brother. I just didn't want to say it because:
2) a lot of people are surprisingly uncomfortable with the idea of Mario being a new character instead of one of the already established ones.
Spoiler for Spoiler for Steins;Gate:
I wouldn't care except for Steins;Gate, lol. Steins;Gate has won me over to the concept that important roles within the narrative should be filled by important already-there characters, and not brand new ones.
Just thought I should explain why my preference is what it is here.