In an interview with Watch Impress Ryutaro Nonoka, the producer of Valkyria Chronicles, explains how Vyse the Bold and Aika got added into the PlayStation 3 game. The director of Valkyria Chronicles, Shuntaro Tanaka, is also the director of Skies of Arcadia. Since the settings for both games felt compatible mesh of worlds occurred and fan service was happened.
Nonoka who had a leading role in Nightshade jokes that adding Hibana, the star of the PlayStation 2 game, would be out of place. The same goes with characters from the Dreamcast game, Guru Guru Onsen.
Wait… there was an Onsen game?