The guy could be the girl's biological dad or her stepdad & something happened to her mom, like the mom didn't want to be a parent & ran off or she was not in a position to raise the kid or the mom died. Guys who want kids might could adopt but that is generally easier & simpler for women. Guys could try adopting directly from a pregnant woman who cant or does not want to raise the kid, like a pregnant teen who wants to focus on her schooling & might get thrown out for keeping the kid but does not want to get an abortion. The guy could pay her medical bills so her & her family doesn't have to worry about em. I've heard of women adopting kids like that instead of going through the orphanage & social worker route & I'd imagine that a single guy should be able to as well. A single guy wanting to adopt a girl would be concerning to orphanages & social workers unless he's related to her, like she's the daughter of his sister & something happened to his sister.
I'm NOT recommending this but I've heard of cases of teen girls moving in with adult guys they were in a relationship with because the girl's parents had major problems or she got kicked out or something. In some of those cases the couple got married or the girl got emancipated. The good guys acted kinda like loving dad's with em. This may sound very messed up but I felt kinda like a dad at times in all 3 of my relationships, especially my 1st & then my current cuz they had various issues they were dealing with. My current gf feels kinda like a mom with me at times thou(she kinda experiences the mom feeling some but she doesn't feel like that to me) so I guess it balances out That's enough for me to know I should NOT have kids.