Warriors Orochi 3 is coming to PS3 and Vita in an HD upgrade called Warriors Orochi 3 Ultimate. Aside from the graphical upgrades, the game is getting some crossover characters like Kasumi from Dead or Alive and Sterk from Atelier Rorona.
The story behind Sterk joining the game is a bit odd, he returned home after helping Rorona on her mission but go gobbled up by a space-time distortion where he got transported to the same world as the Warriors Orochi universe.
So, just like Yoda and Darth Vader in Soul Calibur, Sterk is in Warriors Orochi via a space-time distortion. I imagine the same excuse is used to throw Kasumi in there, aside from her being there for eye-candy.