Quiet loners are withdrawn — that's a word for people who keep to themselves to such an extent that it seems a little unhealthy.
We can all be withdrawn sometimes, especially when we're sad or depressed about something. Some people get withdrawn when they're angry. A withdrawn person can be hard to find, and even if they're around, hard to talk to — withdrawn people don't like to talk, or they try to talk as little as possible. A person who is withdrawn from the rest of the world for a long period of time is a recluse — an extremely solitary, withdrawn person.