This question leads me to believe there are a limited number of people in the company. With that said, and I mean this sincerely; why do you need a title? If you are a key person in an early stage start-up what will the title do for you? Will it pigeon hole you into a specific role or skill set when you are responsible for so much more?
One more question; Did you mean the right title for [responsibilities] in a startup? If not, please explain what a "startup title" is?
That's a loaded question. I've been in a few start-ups (and some right now) where I've wrestled with the exact same question with respect to those functions and others. Some quick thoughts:
If you are talking about a brand new start-up, add a few more responsibilities and you have the CEO. If you are the Founder & CEO and are handing half your desk over to someone else, Co-founder & Chief Marketing and Innovation Officer might work.
A lot also depends on what brand you are creating for yourself. If you want to be seen as new, young, and innovative, you could go with non-traditional titles except the ones reserved for CEO and CFO since probably want to go with more traditional and conservative titles.
I also think about what title would make that person proud, and cause friends and family say "Oh wow!". You could ask the person what title works for them.
A very leader-like think to consider is what would look good on this person's resume should my start-up fail? I've had to think this through. As a start-up leader, I've always felt it was my responsibility to make sure we make it as a business or if we don't, my team members are positioned well for their next jobs. That's where a good title helps.
Happy to chat on a call with you if you need a sounding board.