The Proton Pistol was like a small version of the Proton Pack with Proton Gun. It was slightly less powerful and couldn't hold some larger or more powerful ghosts by itself. Kylie Griffin almost always was the one to use it, along with the Ghost Trap. As noted Kylie has the Proton Pistol plus the Ghost Trap which may be the very reason the Proton Pistol was made. The Ghost Trap is very large and covers a good part of the back of person. So to get a fourth blaster that could be used they made this smaller one.