Aiddon
[10] Knight
SoulCalibur I had Siegfried and Nightmare as two separate characters, because it retold an abridged version of SoulBlade where he was still Siegfried. It's likely because this Nightmare is using SoulCalibur III-V moves that we will also get Siegfried using his proper Siegfried moves which is a lot of what Nightmare was doing in SoulCalibur I-II.
No he's not. At a glance it looks like he's using Nightmare II's moveset, but upon closer inspection he's also clearly using Siegfried's stances, like the Chief Hold, he has a couple of Siegfried moves that Nightmare II did not have. It seems like they're making him a mixture of Nightmare II and Siegfried, which makes sense thematically because Siegfried is possessed by Soul Edge at this point.