Okay kiddo, let me explain who Batman is in the military.
So, in the military there are special people called Special Forces who are kind of like superheroes. They have really cool skills like being able to jump out of airplanes and survive in the wilderness all by themselves.
One of these Special Forces guys got the nickname "Batman" because he was really good at sneaking around without being seen, just like the character from the comic books.
Now, when other soldiers need someone to help them sneak around and do secret missions, they might ask for "Batman" to help them out. And just like in the comics, this "Batman" guy is really good at being a ninja-like soldier and staying hidden from the bad guys.
So basically, "Batman" is a cool nickname for a really talented soldier who is really good at sneaking around and getting things done without anyone knowing.