Saint Germain first appears in Garibaldi Temple. He dresses as a gentleman, complete with elegant top hat, and speaks quickly in a polite British accent. He implores Hector not to pursue Isaac. He is later revealed as an enemy of Zead, as the priest has a violent tussle with him, while Saint Germain uses his time traveling powers to avoid Zead's blows, and comments he shall continue to return until he surrenders.
Later, he falls into a trap organized by Zead and is holed up, stripped of his power, in the Eneomaos Machine Tower. Although not a true villain, he does fight Hector at one point, although not for evil purposes, rather, only to stop him from going further against Isaac.
Upon his defeat, he acknowledges Hector's power and advises him to follow the path set for him. Before he vanishes in his trademark golden portal, he asks Hector to give Zead his regards. Upon completion of the game, St. Germain appears, addressing the player, and talks briefly about destiny and purpose. He often expresses knowledge of the future, and more often still, his words are somewhat ambiguous, especially in his fight against Zead.
Saint Germain is likely a reference to the hero of Chelsea Quinn Yarbro's well-known book series The Saint-Germain Chronicles. In turn, the Count Saint-Germain character is based off a true historical figure, Comte Rakoczy de Saint-Germain. He lived out most of his life in London and was an active occultist, philosopher and composer. Much of his origins are shrouded in mystery. He claimed to be centuries old and a son of Transylvanian prince Francis II Rákóczi.
Saint Germain appearance seems a reference to "William Antonio Zeppeli" from the manga "Jojo's Bizzare Adventures" by Hirohiko Araki: both share the same facial hair style, similar clothing and an iconic top hat. This is likely due to the fact Igarashi is a great fan of Araki's manga series and he referenced it several times in his videogames.
Coincidentally, the union of Romania with Bukovina in 1919 was ratified by a document named the "Treaty of Saint Germain". Bukovina is one of the significant locations in Bram Stoker's 1897 novel, Dracula.
While Saint Germain himself does not appear in Castlevania Judgment, he is alluded to. He appears to be a member of the time traveler organization Aeon belongs to, and is implied to be an individual whom Aeon enlists to fix issues through the timeline.
Aeon's alternate color scheme matches Saint Germain's. It is also possible to unlock a Silk Top Hat and the Angel Continental mustache which, if put on Aeon's alternate costume, will give him an appearance similar to that of Saint Germain's.
Koji Igarashi later revealed through an unofficial interview that Saint Germain is a member of a group he dubbed "time watchers".
He is the only character in the series to break the fourth wall.