How passionate was he about the military? She could have been a girl he was involved w/ before he went into the military. But if you want their child younger, we could say ....
a. Maybe she's a shrink that works for the military, veterans hospital or the equivalent. They may have had this emotional relationship for a while and only had sex once -- a mistake, obviously. She recommended he see another doctor, but he wouldn't b/c it's not easy for him to connect about the PTSD stuff. She stayed, found out she was pregnant ... she could have hidden it for a while and then just said it was someone else's
b. The daughter of a major family who didn't like going to court. She would always bring a book or find something to doodle on while she was there, and talked to no one really unless her parents told her to say hello, which she'd do like it was a chore. Socially awkward but also completely disinterested. Maybe was known for a brief moment as the biggest weirdo that came to court, but was quickly forgotten. Later, maybe she's working as a tattoo artist while she's going to school and she does one of his first tats. They reconnect, wind up talking and having a couple beers. She's a virgin but not a prude, he's attracted to that, they get it on, pass out, he wakes up almost late for training, whacky hijinx ensues, he has to see her later so she can finish the tattoo. Cycle continues. She's distracting him from his shit and it's showing in his performance, so even though he's really feeling for her, he eventually tells her it has to stop. He's gotta focus. She asks if they can still be friends and he says no (cause he'd still be distracted). Fast forward through masters in arts school, maybe she's been commissioned as the royal artist/director of the arts .... wound up around again that way and said nothing about the kid b/c he didn't wanna know? In this scenario, I'm picturing her a bit younger than him by at least 5 years.
c. An intelligence or explosives expert in the military going through basic training with him. Maybe she just vanished one day and no one knew why. It was b/c she was pregnant and got pulled from the program.