She knows how to put a guy in his place
Your ‘alone’ girlfriend is successful enough. She has her dreams, goals and ambitions. She knows how to achieve them. And no doubt she will expect you to have your own purpose in life. Be prepared to be pushed and motivated by such a girlfriend. She will never let you relax and will always keep you move forward. She hates lazy losers.

She genuinely appreciates your existence in her life
If a girl is always alone, she knows what being single is like. She is now in a relationship and this is a new status for her. She feels ready to strengthen the bond and one of the best ways to demonstrate how special you are is by appreciating your presence in her life. She is grateful for your companionship and effort.