"One of the other sources for Spike was an Emperor of Rome called Caligula, who was a sick and twisted man. He had an affair with his half sister - her name was Drusilla. So who knows the history of Dru and Spike!" JM

"He lived in habitual incest with all his sisters and at a large banquet he placed each of them turn below him, while his wife reclined above. Of these he is believed to have violated Drusilla when he was still a minor, and even to have been caught lying with her by his grandmother Antonia, at whose house they were brought up in company. Afterwards, when she was the wife of Lucius Cassius Longinus, an ex-consul, he took her from him and openly treated her as his lawful wife; and when ill, he made her heir to his property and the throne." (Source)

"His parents had obviously failed raising him because according to some scholars Gaius slept with his sister Drusilla at the age of thirteen." (Source)

"He earned the nickname, "Caligula" (which roughly translates to "Little Boots") at age 2, when he appeared in front of the army in a miniature version of the full military outfit including army boots, or "caliga". He hated his nickname, and during his reign he punished anyone whom he caught using it." (Source)

Some accounts say that Caligula was very cruel and vicious with not only Drusilla and his sisters, but also with others. About six months after he became Emperor of Rome he fell very ill and afterwards started to behave strangely and at times 'crazed'. Some think that he might have had a mental illness. (Gathered from my readings. GP)

Gaius Caligula

Caligula

Caligula: From Spoiled Boy to Insane Monarch

Caligula 37 - 41 CE

Coins of the Emperor Caligula (and his biography)

Roman Emperors (with a family tree)

Caligula: Historical Background (with images of two of his busts/statues, as well as info on Drusilla)

Coins with images of Caligula and Drusilla

Several coins with images of Caligula: page 1 and page 2

The 1979 movie titled, 'Caligola' at IMDB.com (US title 'Caligula') with Malcolm McDowell playing Caligula, Teresa Ann Savoy playing Drusilla and Peter O'Toole playing Tiberius.