
As the distinguished veteran, you decide how the child should level up. Should her strength lie in physical prowess; should acuity be her definition? She gains experience by following you around, by you commanding her, by her watching you. Every one of your actions - direct or otherwise - is muscle memory, an impression on her mind. She is a sponge regardless of how you treat her. Will you be encouraging or will you never be satisfied and demand the impossible?
In combat, you can tell her what weapon and style she should use; if you use the same weapon and style she currently has equipped, she will watch your movements, gain more experience points. Or you can put her on point, observe her maneuvers and adjust accordingly. Do you want her to be more offensive or defensive?
The instances where you can temporarily remove her from your party - perhaps you want to complete a personal quest without her, perhaps you want her to be in “study book” mode - there could be a chance that she will disobey and shadow you, and whatever you do when you think you’re alone will give her experience points in the appropriate stats. Does she see you lying, indulging in excessive behavior?

Natalie is handed over to you when she is a teen. When she becomes an adult and the training ends, the relationship continues, mostly because of her - if you treated her like an animal, if you treated her like your daughter, she will have valid reasons; perhaps she feels she can still learn from you, perhaps she see you as a father-figure. Or perhaps it is something romantic. For you maybe; she marries and has a daughter.
Imagine a second act, some fifteen years later, of you and her against the neighboring country that has increased its attacks against your homeland. Your hair has streaks of gray; dark colors dominate Natalie’s wardrobe. You work well together - her attacks compliment yours, so seamless and elegant that it feels like you are controlling two characters at once. Depending on how you trained her, perhaps this time around you are the brute, the accurate cyclops who can smash heavy obstacles while she is the speed demon reckless with her attacks; in the time you two were apart, between the first and second acts, she taught herself a few tricks, like detecting traps.
The beats between the action set pieces are playful and intimate; if these moments felt forced, it would weaken the relationship, their motives. Their consequences.
Imagine the final act is the point where the world and your rhythms fall apart. Natalie questions the motives of both sides, re-evaluates the good guys and bad, and realizes that we have been fighting for the wrong side, that “in this moment, we are the villains in this story.” Yet you have no reason to believe today’s victories will lead to future genocide. You have no reason to doubt your allegiance. Your country has treated you well with treasure chests, and you know that you have helped secure your country’s place as the lone shining tower amid the plains. Natalie is just as passionate as you, but her affections are for the people you call the enemy.
“If you kill me, promise that you will take my daughter someplace far away from here, from what’s to come.”
“She will grow up to kill me.”
“I didn’t plan this. This wasn’t how things were supposed to turn out. We were supposed to be together.”
“I know, kiddo. I imagined the two of us ruling the world or something equally silly.”
“That’s not how I pictured us.”

Her death doesn’t end the war, does not even intimidate others to pick a side. A meaningless battle fought in a vacuum. But was it? You trained her to be the next-generation version of you. Whoever she fought for, they would win. Natalie chose to betray her homeland, the country your parents sent you to so that it could train you to become an officer whose future would play out from eye level but with the powers of a god.
Your country thanks you for sacrificing so much to ensure the war is won by the better half. You accept their rewards, continue to fight the enemy, but your motivation isn’t for love of country. You want to kill the people who brainwashed Natalie.
But as years pass, you begin to notice changes that Natalie spoke of; social and economic shifts that put future generations in danger of a genocide led by the system the people elected to ensure victory. Your equals - the ones responsible for winning the war - are being given too much power. If they demanded, you would have noticed. You now see this was all planned. Patience was their strength.
Perhaps you were blinded by your allegiance, by the treasure chests they offered you. Physically stronger you may have been, but Natalie was always good at detecting traps.