Chapt 1: Single Character On Stage
I am not convinced that starting with a Single Character On Stage is a bad thing. HeartMate started that way, Heart Change, most the Summoning books, and Echoes in Dark (literally, Jikata on stage).
I think it depends on how long you go and what other hooks you might have. In HeartMate, Zanth walks on stage fairly quickly, and in Heart Change, Vinni shows up rather dramatically.
There can be problems if most of the scene is backstory, or the set up goes on too long.
As for me, if books open with dialogue, I tend to get confused -- who is the real protagonist and why should I care about them?
Of course Mentor usually opens with action, prefers that, and chase or explosions. ;), though, now that I think about it, she opened with a single character (victim viewpoint) in what she brought to cririque...though since I didn't notice, it obviously didn't go on too long.
And in critique, it is unlikely that I will be the one having a problem with this. So if you have single character online, check to see when someone else arrives...
May you enjoy all the techniques of your craft today.