A banker is the girl you know you can get with no matter what. She is the one you go for when all else fails, or you simply can't be bothered to make any effort for others. You may have one, or several bankers.
When using a banker you may refer to "opening the bank" or "cashing the cheque"
"There's no one in here, f*ck this, i'm gona have to open the bank"
"Are you and her a thing now?" "Nah she's just a banker mate"