She could be his sister or a cousin. You should ask him who she is. If he says she is just his friend or if he gets angry about you asking, then run for the hills, he may be a manipulator and a player.
If you don’t normally communicate with him, then try to investigate who the girl is through stalking on social media. However do not get caught up into this. Try going out more and having fun and meeting other guys. Don’t let your life be wasted sitting around waiting for that partucular guy. You deserve much more! Good luck!