Going to be honest regarding Nightmare, people seem to just dismiss the other characters from Soul Calibur that are either posterboys or postergirls, most popular ones or the central character.
Its one of the few series where you can make an argument about other characters, a lot of the roles Ryu, Heihachi, Sol have in their games are just divided in SC among various characters.
While the characters from Soul Edge like Mitsurugi, Sophitia, Siegfried, Taki, Voldo, Seung-Mina and Cervantes are important to the series too, here's the big reason why I consider Nightmare the
poster boy. Nightmare isn't just
Nightmare. It is Soul Edge. It is Inferno. Soul Edge has been in the series since the very first game, Soul Edge/Blade. In that game, Soul Edge/Blade was possessing Cervantes. In Soulcalibur 1, 2 and 6, Nightmare was Soul Edge possessing Siegfried. In Soulcalibur 3 and 4, it was Inferno, the spirit of Soul Edge, using the scattered Azure Knight armor. In Soulcalibur 5, Nightmare was Soul Edge possessing
and would later be in the hands of Pyrrha.
Nightmare isn't just a character. Nightmare is
the chosen persona of Soul Edge, the true poster character, the on-going influence and Main Big Bad of the series. Soul Edge is literally the reason for many characters' adventure start in the series.
-Mitsurugi wanted Soul Edge to compete with the rise of firearms.
-Sophitia and Aeon Calcos/Lizardman were tasked to destroy Soul Edge.
-Taki went out to search and destroy Soul Edge because she believed it was affecting her weapon, Mekki-Maru, and influencing her master Toki.
-Hwang was told to find Soul Edge for his country. Yun-suong would follow in his footsteps to try and find Soul Edge to defend his country.
-Voldo was tasked by his master to get Soul Edge.
-Li-long and Setsuka wanted to find Soul Edge as it would've lead them to Mitsurugi who was also searching for it.
-Siegfried wanted Soul Edge to revive his father.
-Cervantes was once a normal man until he came upon Soul Edge. Then he became the Dread Pirate Cervantes.
-Xiang-hua was tasked to find Soul Edge for her country.
-Kilik's temple family was affected by Soul Edge's Evil Seed and he was forced to kill them, and eventually went on a quest to find Soul Edge after Edgemaster trained him.
-Ivy was made to be a host for Soul Edge, when Cervantes was possessed by Soul Edge and ***** a young maiden while possessed. Not only that, but her special weapon, made with alchemy, was fully constructed with Nightmare's help and she owed him for it. And now she literally fights against her fate of being used as a puppet by Soul Edge.
-Astaroth was made to find Soul Edge. And then he was eventually revived using a shard of Soul Edge in the next game.
-After the events of Soul Calibur, Maxi went missing, but was found and healed using a shard of Soul Edge.
-The Evil Seed and shards of Soul Edge had Cassandra try to find and destroy Soul Edge to free her sister from her oath and heal her niece and nephew.
-Talim went on a search to find Soul Edge due to wanting to find where the shards wanted to go to.
-Raphael's quest started because he wanted to use Soul Edge to change the world for Amy.
-Charade was literally Soul Edge shards that became a body.
-Zasalamel wanted Soul Edge AND Soul Calibur to make himself mortal.
-Tira was found by Nightmare/Soul Edge and she follows Soul Edge blindly.
-Pyrrha was literally affected by the shard of Soul Edge in Sophitia when she was a baby, and would not only become Malfested by Soul Edge's influence, but eventually became it's new wielder after Nightmare.
-Patroklos' hatred of the Malfested was taken advantage of by Nightmare and was used by Nightmare to kill innocent people he claimed to be Malfested.
-Algol wielded Soul Edge, and through his many battles with it, it eventually became evil and possessed his son, whom Algol tried to save, but failed. Algol then used a piece of Soul Edge and sacrificed his life to make Soul Calibur.
-And Soul Calibur, and it's spirit, Elysium, were made to stop and/or destroy Soul Edge, no matter the cost. And yet the fact that it was formed by a piece of Soul Edge may have had a possible influence on Soul Calibur's nature and judgement.
-That's not even counting how the Guest characters' roles in the games have them trying to find Soul Edge and either claim it or destroy it. In Necrid's case, he was a warrior that once got ahold of Soul Edge, but got trapped by the Chaos by Inferno. He eventually escaped after Soul Calibur defeated Soul Edge, but found he could not survive outside of the Chaos. So after attacking a bunch of people thinking they were enemies while he was in pain, he found a shard of Soul Edge which softened the pain and then made it his goal to find Soul Edge, stop Inferno and go back into the Chaos with Soul Edge to stop it from affecting anyone else.
Throughout the entire series, Soul Edge has always been the influence and force behind the series. Many characters and their lives were influenced and changed by Soul Edge.
Soul Edge - It was the reason behind the Dread Pirate Cervantes.
Soul Calibur - It was the reason for the Azure Knight and the Evil Seed, an event that forcibly changed and corrupted many people.
Soul Calibur 2 - It's shards scattered across the continent and were found by many people after it's defeat by Xiang-hua and Soul Calibur.
Soul Calibur 3 - Zasalamel brought the Azure Knight remains to Soul Edge, and in turned resurrected Nightmare, in an attempt to bring Soul Edge and Soul Calibur together to make him mortal.
Soul Calibur 4 - The fight between Siegfried and Nightmare brought upon a cataclysmic event that ended up pulling almost all of the Soul Edge shards back to Soul Edge, almost
completing it and bringing it back up to full strength.
Soul Calibur 5 - Soul Edge managed to survive the battle with Siegfried due to managing to get a new host for ahead of time before it could be fully destroyed, allowing it to revive and start it's terror once again.
And not just from a lore standpoint. Soul Edge in Soul Calibur 2 could be wielded as a weapon by EVERY character and Soul Edge was an ultimate weapon for some characters in later games. And it's spirit, Inferno, was also the final boss in 4 games. And it is also used by Strife in Chronicles of the Sword. So the thing is that while the Soul Calibur series has alot of characters, none of them have had much of an influence throughout the actual series aside from Siegfried. And he was only a major player because of the fact that Soul Edge used him.
Soul Edge is the true poster character. It has a recognisable design, being a sword with an eye. And it controls a recognizable character in the form of Nightmare, a giant knight in Blue Azure armor with dark corrupted skin and yellow-red eyes. Nightmare is the form it takes to instil fear into the world and is the form it uses to kill people. Even after Soul Edge got a new host that had a different body structure, soul and fought with an entirely different weapon in Soul Calibur V, it didn't go the route that it did from Cervantes to Nightmare. It kept it's form of Nightmare, the Azure Knight, because people still remembered the killings and massacres of the Azure Knight and the heavy weight and terrors it brought on the world.
Soul Edge is the poster character of Soul Calibur. Inferno is it's Spirit and a major boss in the series. Nightmare, however, is the form directly chosen and used by Soul Edge. Nightmare represents Soul Edge.