Yellow is a girl. She first met Red in the Red/Blue/Green saga towards the end. In the Yellow saga however, when Blue got Yellow's help with finding the missing Red, she gave Yellow the straw hat to use as a diguise - I can't remember the actual reason, but I think it was to conceal her indentity from the Elite Four. Later in the Yellow saga, Yellow's hat is knocked off during a flood on Cerise Island, revealing to all the other characters that Yellow really is a girl.