Gilles de Rais
For a game series about a vampire lord, there's surprisingly few actual vampires in the series. Beyond Dracula himself, there are only two vampires to speak of for most of the early game series: Carmilla and Elizabeth Bartley. One could have almost assumed that Dracula was just threated by having other vampires around unless they were extremely loyal to him. Maybe vampires were like cats and only certain vampires could get along together.
That changed, though, with Castlevania for the Nintendo 64. In that game there were suddenly vampire everywhere. Vampire servants made up the household of the villa, other vampires talked the halls (and one had to be fought by Carrie as a boss fight). At the lead of this vampire horde was Gilles de Rais, a sworn servant of Dracula and yet another vampire. As it's stated in the game, Gilles tends to go around making other vampires, so clearly the act of making vampires wasn't all that difficult.
We still have to assume Dracula was pretty territorial though, a trait Gilles didn't share with him. It's the only explanation we have fo why this one game (and its prequel) feature so many more vampires than any other game, before or since, in the series. Gilles must be a people person when it comes to other vamps. A real social chap, if you will.
Castlevania: Legacy of Darkness
Born in 1406, Gilles de Rais was a French baron. He was also an unqualified psychopath, a serial killer of children convicted of many heinous crimes. Unfortunately for the rest of the world, instead of being killed he was turned into a vampire and survived his eventual execution by hanging (since you really can't hang something that doesn't breathe). Sneaking away from the site of his grave, Gilles then spent the following centuries moving about, killing as he saw fit, before falling unde the sway of Dracula's power.
When Dracula was defeated by Morris Baldwin and his trainees (if you accept Circle of the Moon as canon), Gilles plotted with Death and Actrise, two of Dracula's surviving lieutenants, on a way to bring the dark lord back. Their plan was multi-staged and first involved luring a being of significant power to the castle. Using a rival lycanthrope, Ortega, they had Ada, the adopted human sister of werewolf Cornell, kidnapped which then forced Cornell to pursue the villains to the castle.
With the werewolf nearly in their grasp, the minions then went about testing his resolve, first with Giles confronting the lycanthrope at the entrance to the Villa (and building in the outer grounds of the castle). They then had their pawn, Ortega, delay the werewolf at every turn, stalling Cornell's arrival at the top of the castle until the ceremony was ready (and Cornell has been sufficiently weakened). Gilles first threw himself at Cornell to try and delay the werewolf that much longer and weaken him as much as possible. Then a resurrected Dracula confronted the werewolf (although the vampire lord wasn't at full strength and wasn't even fully back from the dead yet).
The battle ended with Cornell sacrificing his werewolf side to, hopefully, defeat Dracula and regain his sister. The two, hero and sister, left the castle united once more. However, the three minions took control of the werewolf's power and used it as fuel for the true resurrection of Dracula.
Gilles de Rais was a real historical figure and a nasty piece of work. Although his character in the N64 Castlevania games wasn't given much of a background story, we took the liberty of working real historical fact into his storyline here to give his page a little extra depth. In the U.S. version of the game, Gilles is treated as little more than a "fake Dracula" and disposed of quickly in most sequences of the game.
Castlevania for the Nintendo 64
We skip ahead eight years and Gilles had been a busy man, going out into the countryside to collect young children, stealing them back to the castle. This attracted the attention of three heroes: Reinhardt Schneider, Carrie Fernandez, and Henry Oldrey. Each hero arrived seperately, but they all had a goal of finding out what was going on at the castle. While Reinhardt and Carrie dived deep into the castle itself on a quest to confront Dracula, Henry went about finding as many of the children as he could, helping them escape from Gilles and his evil plans.
The children, though, were essential and one wasn't able to flee the castle: Malus. Using this boy as a host, the three minions were able to place the soul of Dracula within the boy's body. All that was then needed was for the power to mature so Dracula could resurrect fully. Gilles oonce again put himself forward to act as a last line of defense while the dark lord recovered. When the heroes came to the top of the castle, Giles (in the guise of Dracula), waited for the heroes. He took them on, using attacks not unlike Dracula's own, hoping to turn them away or, at the very least, make them think that with his death they'd actually defeated the real Dracula.
Unfortunately for Gilles, the heroes didn't buy his ruse. After defeating the false Dracula, the heroes then went on to confront the real vampire lord (now full resurrected and having contorted poor Malus's body into his own dark visage). The heroes battled bravely and, as always, Dracula was eventually defeated once more, sent back to his grave to await the next time his minions could resurrect him.
A major plotline for the N64 Castlevania was that Dracula's minions had kidnapped a bunch of children to use as possible hosts for the dark lord's power. Considering that Gilles was a serial killer who went after children, this plotline not only makes creepy sense, but I think we can also fill in the blanks as to what would have happened to the kids if, in his storyline (which was a bonus mode in Legacy), Henry hadn't rescued all of them.
It's worth noting that if the players take too long to get to the last stage of the game, they will have to fight a vampire version of one of the NPCs they met earlier (Charlie Vincent). Doing so triggers only a single further fight against Gilles de Rais. Here, his ruse of pretending to be Dracula pays off, and the characters then leave the castle thinking they did in fact defeat Dracula. They then play the bad ending which, in each case, has an eerie Malus leave the castle with them, secretly acting as the host for the vampire lord they failed to defeat.
As noted above, Gilles De Rais was a real person, a French baron and also a convicted serial killer of between 80 and 200 children (although some theories propose murders as high as 600). Although there are some doubts as to if he really did commit the heinous crimes he was accused of, most historians agree that he likely was a cold-blooded killer.
Although not normally associated with vampire mythology, there are some stories that indicate he might have been a lapsed Catholic and worshipper of nature spirits. This could have been twisted a trial to make him out to be a warlock. This could have been what interested Konami in using him as a vampire in their game.
Fighting Against Gilles:
By release order of the games (and not the in-series chronology), the first time players will face off against Gilles de Rais is in his "fake Dracula" guise at the top of the castle in the N64 Castlevania 64. The vampire will act much like a real Dracula, teleporting around the room (in an attempt to get behind the player). When he appears he will unleash one of sevreal attacks, including a homing fire attack, an attempt to suck the hero in and drain their blood (potentially also giving the hero the Vampire status effect), a sustained fiery attack, or two shockwaves (low and high) for the players to dodge. As a Dracula-like being, Giles has to be hit in the head for him to take damage.
His next encounter in Legacy of Darkness is seen only by Cornell (and it technically takes place first in the series' story). Here, the vampire fiend will attack a lot like the lesser vampires that inhabit the Villa. Giles will run around (quite randmly at times) all in attempt to get close to the hero so he can use one of three attacks: slashing with his claws, biting Cornell and draining blood (which can, again, potentially inflict the Vampire status effect), a shooting out fiery blasts. While you can try to dodge and feint, we've found that Gilles here (as with so many bosses in the game) can be handled easily just by running at him, jumping around a lot to "dodge", and slashing a lot with the primary and secondary attacks of the hero. Gilles should be defeated quickly with this silly, surprisingly effective, method.