I mean, why wouldn’t you?! I would totally pay money for this novel–just saying. 
At least for me, it’s hard to not put some piece of myself into a (main) character when I’m writing prose fiction. As someone with anxiety, I’ve noticed that some of my short stories are a kind of desensitization process where I put a character in [terrible situation] because it’s something I would hate to experience myself. So in that sense, some of my characters are 100% supposed to be versions of myself. (Even if, uh, they usually don’t share my gender or anything else about me!)
Interestingly, with interactive fiction I find I don’t approach characters this way at all. Probably because I’m usually writing “the player” in mind as an amorphous anybody, and instead I’m thinking “who would be interesting to interact with?” or “who would force the player to advance the story the most?”
In these cases, I usually pull from archetypes. So I might start with “the big bad doesn’t want to be evil” or “the spoiled rich brat who’s emotionally traumatized” and then explore what that would mean in the world I’m thinking about. (Both of those are from The Bread Must Rise.)
The “Which comes first?” question is an interesting one. For me, I almost always get my initial story ideas as a kind of mind picture of a person in an extremely specific situation–doing some kind of action or making some kind of decision. After that, I tend to work backwards and create the plot and world and characters all at once. So for me, character and plot usually inform one another! (Not always, though. Sometimes my mind picture concepts are more like movie voiceover guy’s “In a world… where [interesting thing]…” And sometimes I even get them from listening to a talk at an academic library conference, ha.)
I think this is part of why so much of the fiction I write is flash fiction. I sometimes have a hard time moving very far past that initial concept, because it’s really what interests me the most. So it’s much more fun for me to write 750 words of a fictional guide for sentient ships who want to change their names with no characters as such at all than it would be to turn that concept into a whole novel with fully developed characters, conflict and plot and etc.
Over the years as a writer, though, I’ve learned that everybody tends to have a wildlly different approach to everything, and none of them are wrong. It’s just that when people say “this is the way to do it” there’s usually an implied for me at the end of it. So I could totally see a process where someone does just come up with characters and then builds a world way later. Or where someone comes up with the world and then builds characters way later.
Fonda Lee, for example, talks about coming up with characters by thinking about people she would want to date. 
That definitely wouldn’t work for me at all!! But it seems to work out great for her, considering the success of Jade City and its sequels.
I also remember reading an article somewhere once about writers who said they didn’t tell their characters what to do. Instead, they concieved of their characters as “real people” who were making their own decisions, and they were just writing them down. That idea is so foreign to me that I think it’s bizarre–but these were all professional novelists and respected writers, so clearly that approach works really well for some people!
but sometimes they do end up with some similarities to myself. Write what you know and all that. Currently, it’s multilingual Nordic people.
