Yes, you can have a "harem" of sorts. You can start routes with multiple characters. The only time you're really locked out of anything is when you decide to share a girl with someone or not. Just in case, read up the guide to make sure you don't accidentally lock yourself out with anyone: https://docs.google.com/document/d/1LkXWll0_8kVL23hHWWySoRwKL_4dyluPHeLQsnhvwuE/...