Agrippina the Elder
Nero's father was the adopted son of Tiberius, who was himself the adoptive son of Augustus, whose adoptions were the result of the death of Gaius Caesar in February AD 4. Gaius, who was the heir of Augustus, had died of illness in Syria. Germanicus was for some time considered the new heir by Augustus, but Augustus later decided in favour of his stepson Tiberius. As a result, in June AD 4, Augustus adopted Tiberius on the condition that Tiberius first adopt Germanicus. As a corollary to the adoption, Germanicus was wed to his second cousin Agrippina the Elder the following year.