- Given her youthful appearance not only in-game, but before arriving there, perhaps she was very young when Masato was born. If she had him at 18, and he is 15, she would be only 33 years old. It's possible (but not likely) she might have been even younger.
The Final Boss in fact,(for Masato, that is), as the game itself is designed with "dysfunctional households" in mind and even if it is on the opposite side of the gray spectrum, Mamako's relationship with Masato is just as harmful and abusive as any other beta-player's are with their mothers. Her being overpowered from the get-go is in line with other "bosses"(other mothers) who also are overpowered in one way or another similar to her. The only reason she appears as an "angel" to the others as "demons", is that she clings to Masato (The "Protagonist") the majority of the time, making her "good" by association.
Even so, at one point, in order to escape the game, Masato will eventually have to grow a backbone and face her for real, in order to gain his own independence as an individual in the process.
Shirase had Porta accompany the party to help things go over smoothly, it's why she always has something useful on hand for every situation and why Porta's mom hasn't been seen or hasn't been mentioned yet.
- Shirase does casual mention that she has daughter one of the times she is trolling Masato.
- This would also save a lot on baby sitting fees not to mention letting Shirase spent some time with her daughter even at work!
- This may be jossed now since Porta's real name has been revealed. Also its heavily hinted at in LN#5 that Porta is somehow related to the Libere Rebellion in that the Fearless Leader of said rebellion demands their followers protect Porta. Porta's real name is Moeko Hotta.
Either she married Masato's father after the real mother vanished or died and Masato was so young at the time that he came to believe that Mamako was the actual mother. Or, Masato was adopted into the family at a young age and came to believe her to be his mom once actual memories formed, but neither of them are actually blood-related to one another. This could explain why Shirase seems so hellbent on trolling Masato with perving up Mamako in front of him in any way shape or form and the discord between the lingering feeling in his mind that Mamako might not be his actual mother, would qualify him and Mamako for the game, which seems to focus on dysfunctional household as its target group in order to mend them together again. As he plays along in the game, he's about to get the bomb of "Adoption/Mamako is actually his stepmother" dropped on him, and the outcome depends on how well he got along with Mamako through the game before it happens.
In terms of game mechanics, Masato is a character who has horrible base power, but extremely high growth potential, while Mamako has comically high base power but less room for growth (yes, even with all her insane skills). The entire key to his power is based on how close he is to his mother; The better his relationship with her gets, the stronger he'll become. After all, what parent doesn't want to see their child grow up to do even better than they did?
He will travel back in time after he falls in love with Mamako, and father himself before returning to the present. That's why we haven't seen the father/husband yet.
This may yet be the case due to her vague responses of her background, and of the outcome of the trap in the final episode from her perspective when asked by Masato. She also seems a bit too aware of all the mother movesets Mamako uses.
They look pretty much the same and Satomi Arai even narrated at one point
Porta's mother is unavailable (probably due to some suitably angsty and sad reason) and so Porta's father came in the game with her. Porta may be too embarrassed to admit this or has been asked by Shirase not to let anyone know that fathers could play too.