In a behind-the-scenes video, Soulcalibur V director and producer, Daishi Odashima and Hisaharu Tago, give some insight into how they went about deciding upon new characters for the game.
In the case of Patroklos and Pyrrha, the two were chosen because they were Sophitia’s son and daughter. Sophitia, having a connection with the Soul Edge and Soul Calibur, plays an important role in the series, and her children gave the development team a reason to continue the Soulcalibur story.
The decision to include Natsu came from deciding that Taki would likely be too old to be a ninja anymore. In Odashima’s mind, you can only be a ninja up to a certain age, after which you retire from active duty and focus on training younger ninjas. While Taki has retired, she’s still watching over Natsu, he adds.
You can watch the video in its entirety above. More behind-the-scenes videos are included in the collector’s edition of Soulcalibur V.