Over the course of it's long life span the Castlevania series has taken some very strange narrative pathways, although arguably nothing was weirder than when time travel was introduced into the series. Although an attempt was made way back on the Dreamcast, with Castlevania: Resurrection, that game was canceled and time travel wasn't brought in just then. No, it was take another few years before series manager, IGA, decided that what the series really needed was time travel.
Specifically the implication was that there was a corps or people who went up and down the timeline maintaining order, attempting to ensure that time ran its proper course. These "time agents" showed up twice in the series with their first appearance in Castlevania: Curse of Darkness. There, a mysterious man, Saint Germain, with strange powers and abilities, as well as knowledge of the future, stands in the path of the heroes, Hector. Why? What was his plan? What was it all building to?
Well, that remains unclear. The next time we saw a time agent it was Aeon and he was working in the very far future of Judgment. Clearly there was some purpose to their madness, some need to save the world from those who would seek to destroy it, but, again, little explanation for who they were or what they were really planning, long term, was ever discussed. But hey, the time agents sure were snappy dressers, eh?
Castlevania: Curse of Darkness
Appearing mysterious early in Hector's quest to find, and kill, the evil Issac, Saint Germain gets in the way of the hero, attempting to warn him off. There are any number of ways to view his interference: maybe he foresaw a future where Hector became a mindless host for the power of Dracula (something Issac was, indeed, plotting). Or maybe there were bigger plans for Hector that involved him avoiding his quest for revenge entirely. Whatever the case, Saint Germain kept it cryptic. After fighting Zead (aka Death), revealing his powers to manipulate time, Saint Germain disappeared with promises of his return.
And return he did, much later in Hector's quest, after the time traveler had been captured by Zead. When once again confronted with Hector, Saint Germain tried to stop the hero from moving forward (one way or another, for the sake of the timeline). When Hector seems adamant in his intentions, Saint Germain challenges him to a fight. Hector, though, wins the fight and Saint Germain finally acknowledges that Hector must follow his path, one way or another. Then the time traveler disappears once more, allowing Hector to move forward to his inevitable fight with Issac.
Saint Germain appears one more time, at the end of Curse of Darkness. This time he doesn't address Hector but, instead, turns to the player and talks directly to them (making him the only character to truly break the fourth wall in the Castlevania series). Here he talks about fate and destiny, acknowledging events like Dracula's eventual death in 1999 AD, cementing the fact that he's a time traveler. Then he fades away, never to be seen again.
A different version of Saint Germain shows up for the third season of the Netflix Castlevania series. Here he's an older man, beaten down by events in his life. He's spending time in Lindenfeld (the town Trevor Belmont and Sypha Belnades arrive at, kicking off much of their adventure in the season), studying the books and works of art at the local priory. This is a religious building, it should be noted, that had been taken over by evil monks, led by Prior Sala, who had, months before, forsaken God and had begun worshiping a hulking Night Creature that had stumbled into the city.
After a chance encounter with both Trevor and Sypha, Saint Germain explained why he was in Lindenfeld, and it wasn't just to study books. He'd been using a magical artifact he'd picked up before, one with a connection to a magic inter-dimensional place called the Infinite Corridor. He'd had a lover who had gotten lost in the corridor and he was desperate to get her back. As he followed that device he'd been led to Lindenfeld and, he suspected, another portal to the corridor was going to open under the priory. He feared what the monks could do if they somehow managed to open that portal.
His fears were more or less confirmed when he managed to sneak into the crypts under the priory and he found the hulking Night Creature, the Visitor, trussed up on a cross with alchemical symbols (in blood) smeared in front of it. He immediately ran to the town leader, the Judge to inform them, and they called in Trevor and Sypha as well. A plan was hatched to stop the monks from casting their spell as soon as possible.
Sadly, before they could act, the monks enacted their plan. They performed a ceremony that burned all the townspeople (saving those heroes that were already stalking up to the priory), sending their souls into the waiting maw of the Visitor. This gave the creature the power it needed to open the gateway and the beasts of Hell came pouring out. Trevor, Sypha, and Saint Germain went into the priory, with Trevor and Sypha fighting while Saint Germain (who was not a warrior) was protected by a shield of ice, following behind.
When they got into the crypts, Trevor and Sypha continued fighting the dread beasts while Saint Germain went to work on the portal. He kept it from opening further, but it was Trevor defeating the Visitor that allowed the portal to close. Before it was fully closed he leapt into the portal to go in search of his lady fare, with promises that he'd see the heroes again, one day.
Like his video game counterpart, Saint Germain has some knowledge of future events (making comments about things like "toilet paper"), although he doesn't seem to be able to freely time travel the way the video game Saint Germain could. His knowledge, instead, comes from his journeys in the corridor and the places, and time periods, he's seen there.
Fighting Against Saint Germain:
Although you see Saint Germain battle Zead early in the game, there's only one official battle the player actually participates in, and this comes much later. In the fight, Saint Germain will teleport around, trying to slash at Hector from behind. Watch out because his sword slashes can sometimes cause the poison status effect, further damaging you. After he slashes, though, he'll take a pause so you can layer on some damage. He can also freeze time to try and get an attack in on you. And he can cause radial damage around him, forcing the player to jump and retreat to avoid damage. Once he reaches half health he'll start spamming his more annoying attacks, causing time to speed up which means Saint Germain can attack more often. He'll also summon a thorny vine that will craw along the ground and attack the player.
And then, obnoxiously, once defeated he'll reverse time, refilling his health bar once, and the fight will start again. At this point, on top of everything else, he can bring out a gun and shoot three times at you. He can really layer on the damage if you aren't careful. Thankfully, after three times through his health bar, Saint Germain will stop and you can continue your progress through the game.