As long as you're getting the benefits of mentoring and career guidance, don't get too hung up on a title your boss may not be able to give
My boss treats me as her No. 2 in very obvious ways. I sit in for her at meetings when she's out of town, and she has sent me to customer visits on her behalf many times. She has asked me to handle staff development issues, and she gives me numerous signs that I am for all intents and purposes her first mate. I want to ask for a title change to assistant manager, but no other department in our company has a person in that role. Is there a risk in asking for a position that doesn't exist? Any tips on how to proceed?
First of all, congratulations on the mentoring you're getting from your boss. That is outstanding.
Your instincts are right about hesitating to ask for something your boss may not be able to give. But really, the title is secondary, and I wouldn't get too hung up on it. If assistant manager isn't a title used in your company, then your manager may not want to set a precedent, and she also may not have the authority to create that new role. Rather, I'd think about what's important for you, long-term. Do you want to replace your manager when she gets promoted herself, or leaves the company? Then tell her. Talk with her about your career plans. Ask her what she sees as likely next steps for you. Ask her what she sees as the next role for you and what it will take for you to get there.
The best career-planning conversations are open-ended ones that let you discover exactly what your manager considers your strengths and developmental needs, how she sees you moving ahead in your career, and how she sees herself helping you get there. Unless there's a compelling argument for how an assistant manager title would help you right now—seeing as how you're already getting the respect and the assignments the title would normally confer—is there any strong rationale for pursuing it?
If your interest is mainly financial, talk with your manager about salary progression in the same conversation where you two discuss career advancement. Asking for information ("What are your views on where I'm headed in this company?") is generally a better move than asking for a bigger title for the work you're already doing. Your chances of leaving the room happy are greater, too.