John "Johnny" Roman DiMera is the son of Sami Brady and EJ DiMera. He is the fraternal twin of Allie Horton, whose father is Lucas Horton. He has two other siblings, Will Horton (materal half-brother) and Sydney DiMera. He is often referred to as 'Giovanni' (the Italian version of Johnny) by his paternal grandfather Stefano DiMera. He got his first name is derived from his step-grandfather John Black and his middle name from his maternal grandfather Roman Brady.
Johnny was originally believed to be the son of Lucas Horton, but after the child's birth, E.J. begins to suspect that the fraternal twins might have different fathers because Johnny does not look like his sister. E.J.'s suspicions are correct. They do a second blood test that reveals E.J. to be the father. Johnny's paternal grandfather, Stefano DiMera was pushing to meet his grandson and insisting that E.J. raise Johnny as a DiMera against Sami's wishes. E.J. first resists this notion because he does not want to betray his ex-wife, but since finding out that, Grace was "his" daughter he agrees to raise Johnny as a DiMera against his ex-wife's wishes. Johnny has one older maternal half-brother, William Reed "Will" Horton, twin maternal half-sister, Alice Caroline "Ally" Horton, and one sister, Sydney Anne DiMera. Sami has allowed Johnny to live with his father, Nicole Walker and his grandfather in the DiMera mansion, while under the protection of the federal government as a material witness to a murder. Johnny had cancer in one eye and got it removed and replaced with a glass one.