I'm not sure we invent characters. They sometimes take on lives of their own. If we write, we use our imaginations - unless we are writing non-fiction.
When writing fiction, my characters are sometimes based on ideas I've had from my own childhood. In some of my original and unpublished work (writing that lives in the many drawers of my cupboard and is probably never going to see the light of day) characters just seem to develop on their own but sometimes a newspaper article or person is going to create a spark in my mind. There is an image I had of a librarian from a film - sort of generic - that I incorporated in Pride & Princesses. She is not based on a teacher I had at school but on a teacher I imagined from all the different kinds of teachers I met throughout my schooling! Sometimes I make characters do things I would never do in real life - but I know other people might do. For example, I know girls who were very nice at school to people's faces but behind their backs they said mean things. None of us is perfect and sometimes people change when they grow up. So, I try to find something about the not-so-nice character that is good, that might not be obvious and let the reader in on that secret. As a writer, I really like the mean girls in Pride & Princesses even though they are not very nice. Teegan, Tory, Brooke and Freya were so much fun to draw in my mind and they have a desire to improve (sort of) by the middle of Pride & Princesses, so I tried to focus on that element of their personalities.
Some writers say that all the characters they write have some of themselves in them. Maybe that's true, sometimes. Most characters like most people are not all good or all bad. They have different qualities and it's in those differences that we create characters.
When I was writing the modern vampire version of Wuthering Heights, I related far more to the Heathcliffe character than to the Catherine character and I'm not sure why. I tend to relate to female characters (with the exception of Ponyboy Curtis in The Outsiders). Heathcliffe was just a strong character to me in the original novel and I never really understood why Cathy Earnshaw didn't marry him instead of Linton.
That's just me. Writing is very personal and the minute you start to think other people are reading what you write, it becomes tough to write honestly. Because 'other people are reading' becomes 'other people are judging.' You have to be honest with yourself and it helps to have a strong sense of yourself. It's a cliche because it's true, lovelies. Now go to it & keep on writing (or reading... or, like me, sneaking out for a cup of tea and something yummy to eat very soon!)