Salem's residents originally assumed Belle to be Marlena and Roman's daughter. However, Belle was conceived in a weak moment for Marlena when she made love to John Black. Sami knew of Belle's true paternity, but she kept it secret for several months. Sami even went as far as kidnapping Belle and trying to have her adopted. Stefano DiMera eventually spilled the beans to Roman after reading Sami's diary.
Belle is currently in her formidable teen years. What remains to be seen is if she will take after her duplicitous elder sister Sami...