I like bullying my girlfriends 12yo son
Why would you bully someone so young?
I think he's an immature little brat.
Instead of bullying him, you should try and show him how to be less immature and bratty.  Instead of being a bully, be a role model!
I never really thought about how I could be an important person to him.
You do have a choice to be a good an important person to this child and that would be more memorable to you I am sure.