You are Robin’s biological brother! She escaped Ohara with you when you were a baby. Yes, you were her younger brother. When Marines found her at the next island while she had left you home, she had to leave without you. The people she trusted to take care of both of you started to take care of you. Later, two years after the Marineford war. Robin is 30. She still remembers you. Until they dock on a random island and Robin sees someone that looks exactly like you but grown up.