She is your best friend. Romance is important, but what really connects people is mutual friendship. Your girlfriend has to be this person you want to message first when something funny happens, or when you have a problem. She is not just your love, but also your closest and most supportive friend, who you can trust with your life.
She accepts and indulges every guilty pleasure of yours. She knows the weird things about you that do not make a lot of sense. For instance, she is aware of your love for a certain type of cake or walking around home wearing ridiculous pants. And she does not make fun of you or scold you for that but supports these things wholeheartedly.
You always want to protect each other. Even if she is perfectly capable of standing up for herself, and there are barely any situations when she needs your help, you just cannot contain your protectiveness towards her.