Marcus Agrippa--In I Claudius TV series, Marcellus and Agrippa are 2 difft individuals--

but Marcellus is the son in law of Augustus and the husband of Julia, the only daughter of Augustus

https://en.wikipedia.org/wiki/Marcus_Vipsanius_Agrippa

Graves has depicted the characters so vividly that the "myth" is now stronger than the revionist views--

After all he based his work on ancient sources.