The real Mona Lisa is thought to be Lisa de Giocondo, who is the wife of Francesco del Giocondo. It is believed that he is the one who commissioned to have her portrait done after their baby was born.
Many people assume that she and Da Vinci had an affair, but there was never any proof of that ever happening. After he stopped working on this painting he decided to keep it for himself, also it was never fully completed.
It is believed that at this sitting she was about 24 years old and was never thought to be a beautiful young woman. In my opinion, at that time folks were clearly blind to what true beauty this young woman really was.
Her smile has captivated many and has become the hallmark of Da Vinci's style. No one will truly know who she really is, but will be captivated by her beauty and smile for many more years to come.