(not as a series but as two stories with two different themes)? What was your reason? Whether it was because, you loved the character or you used him/her due to another purpose?
It would depend on what you mean by "similar". For instance, two characters who were male, military, and competent, would be similar, but so would every MC soldier in every war novel in history. If you mean almost indistinguishable save for name, then no.
Yes. The trouble is the stories branched off each other. I started with one story about an arrogant school teacher who starts an affair with a very cold dangerous girl, or so the reader is lead to believe - he may in fact be losing his mind and clouding what their relationship really is. After almost finishing it I wasn't quite pleased with it but the characters continued to bug me. I came up with another story though the settings remained practically the same - A man seeking revenge contacts an underground group that helps people exact revenge but he is paired with a young girl who is seeking revenge as well when their contact is killed. And they discover they could be after the same person. They wind up in a small town school setting - same as the first novel. The very cold clever young girl - is the same character. The man though was altered from arrogant and showoff-y to withdrawn and bitter.