I’d like to give an answer to this question, but the problem is that it actually encompasses a wide range of backgrounds. All of which can create significantly different personality types. Here’s a short list:
The child of uber-environmentalists or hippies, spent their weekends camping, learning the wonders of nature, and their weeks practicing Tai Chi.
The child of extreme survivalists, who spent their childhood learning self-sufficiency in the wilderness and how to drill a skull with their hunting rifle at a hundred yards.
A Boy Scout (or Girl Scout depending on merit badges), especially one who grew up in Montana, Wyoming, or anywhere that has no sensitivity to guns. (There’s a reason why an Eagle Scout gets an automatic rank bump when they enlist in the US Military.)
The kid who got boxed up and sent off to a Military Academy or Boot Camp.
The upper-middle class kid whose parents shoved them into every single martial arts program and or camping/environmentalist program in order to get them out of the house and out from underfoot.
The same upper-middle class background as the above kid, except the child did it to escape their parents.
The kid who grew up on the streets of a major metropolitan city and had to scrounge for every meal until they learned boxing after landing in Juvie.
The kid who grew up on the streets in a major metropolitan city and had to scrounging for every meal until they were discovered and taken off the streets by a kindly boxing coach.
This is only a small number of the unlimited possibilities and even within the subset, the personalities created while not unique are many.
The important truth to remember when thinking about personalities for your character is that personalities are developed through individual experiences. Change a minute detail like the kid had an instructor who liked them to the kid had an instructor who ignored them and you change the experience thus changing the effect on the personality.
A specific kind of training can develop an outlook the individual adopts. An outlook is the way someone views and sees the world around them, but training is only part of the equation. Home life, friendships, school experiences, religious background, political background, these are all aspects of our experiences informing who we are and how we look at the world. They also inform what we choose to do with the skill sets we have.
It’s also important to remember that the training outlooks for each individual martial art is different. They take on the aspects of what they were or currently are meant for and the values of the culture they come from. Taekwondo, for example, pushes the importance of community to it’s trainees. Giving back is the watch word, we have a responsibility to take what we know, what we’ve learned from our training, and use it to benefit others (not in the superhero sense). This mostly takes the form of mentoring other students in the dojang, but it also encompasses community service and other projects outside the school. In fact, in order to progress up the belt ranks, each student must write an essay as part of their final exam detailing what their understanding of their training is.
However, a child who has always had difficulty making friends or comes from difficult family circumstances may adopt these tenants differently from a child who has loving parents and lots of friends outside the school.
Think about what kind of survival training and fighting training your teenager had. What were they being trained to do? Was it self-defense? Purely for recreation? Health and fitness? Hunting monsters? Post-apocalypse survival? Why were they learning this in the first place? What sort of community surrounded them while they were growing up? Was it an insular one? Did they have regular access to individuals outside their family group? How present were their parents? Is this a family thing or something they decided to pursue on their own?
Once you’ve outlined their background and figured out what kind of training you want them to have, you can then fill in the blanks by limiting your search to martial combat and skill sets which cover what you’re looking for. This limits your search so it becomes easier. Try not to have a concrete idea in mind, otherwise you’ll end up rejecting the real world information you dig up because it doesn’t exactly match the idea in your head. Once you’ve absorbed the knowledge, then you can route back and get the character you’re looking for (or maybe one you weren’t). You’ll have a better understanding of the experiences they might have been through and thus a firmer grasp of the sort of personality they could have. By filling in their background, the person starts to become realized.
When doing this with teenagers, you may have to do some jury-rigging if you want them to have a skill set that’s not normally available to a teen with their background such as military grade Systema or military grade Krav Maga. You’ll have to figure out how they got that training and create a firm understanding of why they were trained that way. You’ll also have to accept that this training will change them in ways that are noticeable to other characters around them. People, even kids, are very good at picking out dangerous individuals and avoiding them on a gut level, even if they don’t consciously know why they’re doing it. A kid with this kind of training may also have difficulty relating to others or even seeing other individuals as individuals. Their concept of right to life may be abstract at best and they’ll already know how to push themselves through significant mental hoops in order to justify killing someone.
However, this isn’t a normal background or normal setup for a character or even something normal/average/even bad parents would allow to happen to their child. You’ll have to set up their background to justify it and allow your other characters to act accordingly.