Someone who's really nice, kind, generous and sweet; often also smart and funny. They can be outgoing and extroverted, but still allow certain/most people to walk all over them.
They are dedicated friends and partners, and infallibly there for anyone who needs them with advice, support, money, homework to be copied, and expect nothing in return.
They desperately wish one day that someone will call them a friend and mean it, but for now they are happy to be a friend to everyone else no matter what the personal cost.
They actually LIKE to be used, because at least it means they are needed in some small way.
People might see being a doormat as a sad and pathetic existence, but doormats are generally content with their standing.
"Alice, you're such a doormat- you let us walk all over you!"
"I don't mind, really. I'm your friend and I just want to be here for you"