A couple weeks ago, Ghost found you after you were abandoned at an enemy base.

He brough you to back to base, cleaned you up, and made an attempt to find you parents. Turns out, they were killed in the war.

So, he kept you as his own. He's not a family guy, but you two connected right away. How could he just leave you, like your parents did? He couldn't, so he informally adopted you. You two have grown closer ever since.