As one of the four heroes of Castlevania III: Dracula's Curse, Sypha Belnades would be remembered by fans simply for her place in one of the most revered games in the series, but there's more to Sypha's legacy than one game --it's important to note they various things the character started for the series that contributed to making the name "Belnades" nearly as important the mythology of Castlevania as "Belmont" is.
For starters, Sypha was the first female protagonist in the series. This is a huge deal that should not be downplayed (which, it's worth noting the English manual basically did by mistakenly listing her as a "Mystic Warlord"). it's not as though there were many female characters in the NES landscape by that point, with Super Mario Bros. 2 and Metroid being the only games we can remember offhand with a female in any kind of playable role (well, unless you count White Mage from Final Fantasy, but we're still unclear if she was a girl or not). TO have another female playable is great for the series, showing a greater diversity for the characters than one might expect.
Of course, then it almost seems like Konami went out of their way to downplay the role of women after this. Very few of the playable characters have been men, and almost all of them have had to be rescued first (Sypha included), or were simply pawns of men (I'm lookin' at you, Order of Ecclesia), or, even worse, are simply kidnapped or killed off to give their men motivation (without being playable at all). Only two characters spring to mind, Carrie Fernandez and Sonia Belmont, who were agents of their own destiny, and even Sonia had to have a man (Alucard) help her from the background... and then she was wiped from continuity altogether. Damn.
So maybe Sypha set some bad precedents as well as some good ones. Still, because of her the series gained a band of female magic users -- all female, mind you, with only one man, Juste Belmont, showing any inclination towards the magical field they dominate (and he got those genes from Sypha). The Belnades clan recurs again and again, agents of good working against Dracula even when the Belmonts are no where to be seen. That's not such a bad legacy at all, really.
Raised from an early age to be a witch, among a coven of witches, Sypha Belnades grew to have great powers of the elemental spirits of the land -- ice, fire, and lightning. When darkness spread through the land, the witches of Wallachia (Sypha included) worked to protect the people and stop the evil from spreading. As elaborated upon in Castlevania: Judgement, the evil vampiress Carmilla sense that these witches would be a threat to the lord Dracula's plans. Spreading rumors and lies about the witches, Carmilla was able to make the villagers dobt the intentions of the witches, and thus began a series of vengeful witch trials -- trials that left most of Sypha's sister's dead, and Sypha alone to wander the countryside.
Eventually Sypha was taken in by the church for protection and training, as they sensed within her the goodness and light she could bring to the land. The Church employed her to battle the forces of darkness and to work against any threats Romania might experience. One such threat came when a dimensional rift was opened by a mystical traveler, Aeon. In the far future, Galamoth (evil villain and rival to Dracula) looked to defeat Dracula in the past and change history so that Galamonth came out on top. Sensing the kind of damage this could do to history (i.e., a lot), Aeon brought heroes and villains together in the rift in hopes that he could find one among them with the power and skill to stop Galamoth's plans... by pitting them against each other in combat (via a fighting tournament, naturally).
Although Sypha would met her eventually companions (see: Castlevania III: Dracula's Curse below), including her eventual husband, Trevor Belmont, Sypha was still early in her timeline and did not realize the relevance of these encounters. She did, however, prove to be a formidable opponent in the tournament, one quite capable of taking on Galamonth and his minions.
(Of course, since it was a fighting game any one of the heroes, or villains, could have proved victorious -- the exact hero to end Galamoth's plans was never determined by Konami, so it's up in the air now if Trevor officially cast the fatal blow or not.)
Castlevania III: Dracula's Curse
Eventually Dracula's power grew and spread, sending the legions of evil once more across the land. The church, realizing this was a threat that had to dealt with quickly, sent their best warriors to the evil vampire's lands to try and defeat him and send his soul to Hell. Sypha joined this group, vowing her own revenge towards the vampire and his minions for their hand in the slaughter of her coven. Unfortunately, Sypha's quest was cut short when she was captured by the Cyclopes and turned to stone (not a power normally attributed to Cylopes, but since it's in the series, we'll let it slide).
Not long after, Trevor Belmont came to Dracula's lands to try his hand at defeating the dark lord. Searching through the swamps around Dracula's castle, Trevor came across the Cyclops, engaging the beast in battle. Upon the Cyclops's defeat the magic that bound Sypha to her statue form dispelled and the sorceress was freed. After a brief discussion, Sypha willing joined with Trevor on his quest against Dracula.
Together, with the aid of the heroic rogue Grant DaNasty and the vampire lord Alucard, Sypha and Trevor were able to make it to Dracula's throne room and take on the vampire. Though the battle was tough, the vampire was defeated. With the rest of her companions, Sypha watched the castle crumble into the sea, confident that, for a time, the land was free of Dracula's dark magic.
Eventually, she and Trevor were married, and her bloodline would go on to influence the heroes of the Belmont clan for generations to come.
To note, Castlevania III: Dracula's Curse did receive one remake, a cellphone-only edition. From what we can find, the game was largely the same as the original, just modified to work on mobile devices.
Castlevania: Symphony of the Night
Officially, in Castlevania: Symphony of the Night, Alucard has to face off against three zombies in the Inverted Coliseum -- three zombies that look like his old friends (from Castlevania III): Trevor Belmont, Sypha Belnades, and Grant DaNasty. This is a nice nod to the previous title, but this isn't considered an official reappearance of Sypha (or the other two) since these are just zombie look a-likes.
Castlevania: Portrait of Ruin
Similarly, in Castlevania: Portrait of Ruin, the Zombie Trio make a second appearance, this time in the Nest of Evil. That area serves as a kind of "best of" for various Castlevania bosses from the Metroidvania era of the series. They don't serve any specific plot points here and, as before, these are just zombie look a-likes and not the real characters brought back to life.
Castlevania (Netflix Series)
Although not seen for the first couple of episodes, Sypha does eventually appear in the Netflix series based on Castlevania III. One among a clan of Speakers, keepers of knowledge (and the magical arts), Sypha actually ventured off on her own to try and find a "great protector" that lived beneath a city (which was under threat of demonic attack due to Dracula and his quest for vengeance against all humanity). While she explored beneath the city she came into an an ancient area that, in many ways, resembled Dracula castle. This was a problem, though, as one of the Dark Lord's monsters was lurking there as well, a cyclops with the power to turn men to stone. Sypha, sadly, was caught in the gaze and turned to statue (not unlike what happened to her in the events leading up to Castlevania III).
Thankfully for Sypha, a hero, Trevor Belmont, came into the underground area at the behest of the elder Speaker (who was also Sypha's grandfather). Trevor fought the cyclops, killing the beast and, thus, freeing Syhpa in the process. The two returned top side and it was through her cajoling (outright guilt, really) that Trevor elected to take up his whip properly and fight against the Church and then, later, Dracula's forces.
As depcited in the show, the Speakers all dress like Sypha in the game series -- namely, long blue cloaks and huge hooods. Sypha keeps her blond hair and Spanish accent (she is a Belnades after all) and gains a fiery attitude in the process (becoming one of the best characters in the series). She augments her steely sense of honor and justice with powerful magics, brandishing blasts of fire and massive walls of ice to help defend the innocent from demons and monsters.
Lords of Shadows History:
Castlevania: Lords of Shadows - Mirror of Fate
In the Lords of Shadow universe, Sypha shows up alongside Trevor. In Mirror of Fate, Sypha Belnades becomes Trevor Belmont's wife, eventually giving birth to Simon Belmont. When Trevor goes off to fight his father, Gabriel Belmont (who has, himself, become the vampire lord Dracula), Sypha is supportive of his decision, only hoping he comes back alive. He does not, in in revenge for the attack on his castle, Dracula sends his minions to attack the town where Sypha and Trevor lived. Sypha was killed in the attack (once again showing that women in Castlevania have it really hard), leaving Simon an orphan.
That's not the last we see of Sypha, though. When Simon eventually tries his hand at taking on Dracula, he gains a spirit guide along the way. This woman (who is actually Sypha) can be summoned to aid him in combat (in a nice nod to her role in Castlevania III).
Playing as Sypha:
Unlike her companions, Sypha is neither a strong fighter nor a very agile one. Additionally, her attack power with her staff is very weak and she has the worst defense of any of the characters in the game. Despite this, though, Sypha can be very useful in battle due to her magic spells (which take the place of sub-weapons for the other heroes). Her ice spell can freeze lesser enemies in place, and these frozen enemies can be used as platforms (for a time). Meanwhile, her lightning spell creates three homing orbs, very useful for tight areas with lots of enemies.
That said, a team-up of Trevor and Sypha is at a disadvantage during the platforming sections. Sypha has neither the benefit of wall climbing, like Grant, nor the ability to fly, like Alucard, leaving her grounded much like Trevor. Players have to decide if her devastating magic is worth the trade off in maneuverability.
Later in the series, the dynamic between the various team members was directly referenced in "Julius Mode" from the DS Castlevania, Dawn of Sorrow. In that unlockable mode, Yoko Belnades (the analog for Sypha in that game) teams up with Julius Belmont (Trevor's analog) and Alucard (as Alucard), to take on Soma Cruz (who has turned to the "dark side" by embracing the Dracula soul within him) to defeat the evil power Soma is threatening to spread across the land. Sadly, there was no Grant analog -- audio clips hidden in the game seem to indicate Hammer, a shopkeep in the game, was to be playable, presumably as the Grant for the team up, but was most likely cut for time.
Although she plays a lot like Sypha, Yoko has a benefit that Sypha lacked -- healing. Her physical attacks, while weak, gave Yoko small boosts to her HP. This edge was sometimes very useful, especially for speed runners who needed an advantage against certain bosses.
Although the Fake Trevor and Fake Grant Alucard encounters in Symphony of the Night (and who later show up in Portrait of Ruin) are based on their respective characters from Castlevania III, Sypha's double plays very differently. She floats around the room, throwing magic at the player. She also has necromantic magic, with the ability to revive Fake Trevor if he is defeated first by the player.
Sypha benefited greatly from her makeover in Castlevania: Judgment. Her magic received a strong boost, making her, offensively, a powerhouse. She also gain a breadth of additional spells (like Gale Force and Disintegrate) that weren't part of the elements traditionally associated with the Belnades clan. Beyond that, she also gained the ability to use traditional sub-weapons, which before this game she's never been able to use. Overall, Sypha proved to be a tough foe (even if, physically, she still wasn't the strongest character).
Finally, a version of Sypha appeared as a background character in Castlevania: Lords of Shadow - Mirror of Fate. Although not a playable character (unlike Trevor and Simon), Sypha does get to lend a hand in battle, acting as one of the spirit guide for Simon, providing a defense boost for him while in combat.