It may seem odd that some fighting styles were updated to match the characters' later appearances even when it doesn't match the time the game takes place during the Soulcalibur era such as Yoshimitsu having two swords or Nightmare wielding Soul Edge with one hand. Then you remember that this is a Continuity Reboot, and thus the old timeline can be changed as needed.
Zasalamel being in the game may seem like a contradiction, as he didn't debut until III and the game only covers the events of I with some elements of II, but it makes sense when you realize he's an enigmatic Chessmaster who's been in the shadows the whole time, and is immortal. Of all the characters introduced later, he might be the easiest to write an Adaptational Early Appearance for.
Soulcalibur VI being a retelling that includes new elements and additions as well as changes from the original makes a lot of sense when you consider what the narrator has always said in the past: Soulcalibur is a tale of souls and swords, eternally retold. Stories that get retold often change, with different interpretations of the tale. This is very much the case.
The Soul Charge visuals are less indicators of a character's morality and more of their powers; all of the non-magical fightersnote Seong Mi-na, Tira, Maxi, Mitsurugi, Raphael, Voldo, Amy and characters with both good and evil powersnote Taki, Yoshimitsu, Siegfried, Azwel or otherwise outworldly note Geralt, 2B use the normal color effect (bluish-white with gold), characters blessed with Soul Calibur or at least have some other holy energy note Sophitia, Xianghua, Talim, Zasalamel, Cassandra use the blue "Holy" effect, while the characters that are malfested or draw their powers from a similar evil source note Kilik, Grøh, Cervantes, Ivy, Nightmare, Astaroth use the red "Evil" effect.
There's a very good reason why Ivy was chosen to be 2B's rival upon the latter's introduction into the game. Ivy's cursed blood renders her functionally immortal, so there is always a chance she'd be able to live to see the distant future 2B hails from. It's a very slim chance, given what that entails for her, but it's still a chance.
There's also the the fact that both are examples of Ms. Fanservice while having white hair; If Ivy wasn't in the game, Zasalamel would have likely been The Rival instead as his Immortality is far less subtle.
2B was the second DLC character and the first post-launch character added; on the season pass beforehand, it reads "DLC2 - Character B. She really could not have been a more fitting announcement for that slot.
During Libra of Soul, one of the antagonists, Valtro, becomes Malfested in order to serve his master better and you last face him when his Malfestation becomes stronger in a cult shrine. Once you go back there in a side quest, you face a fully Malfested creature that is impossible to hurt who dresses and uses the same fighting style as Valtro. And if Azwel's Ultimate Seed would have been sucessful.....