I realized that while narrating to my tulpa, it became somewhat difficult to refer to them if they don't have a name. I thought about it for a moment, and the first name that came to mind was Sarah. It's not a very deep or glamorous story, but it's how she got her name.
I had looked up a list of names online for my second tulpa, and there were many options to consider. I was studying WWII at the time, and I had learned about the British WWII General Bernard "Monty" Montgomery, so when I saw Monty in the list of names, it sounded like to right choice to make.