Dummied Out: G-Red's Evil Counterpart G-Black, was an event only Borg, and the only way to even get access it anymore is through a hack or a glitch. Hacking also allows one to access the Galactic Emperor as a playable Borg, as well as the combined Borgs as a single unit. However the later are barely playable and tend to freeze up during battle.
What Could Have Been: Mana's older◊ sister,◊ Mina,◊ whose role was cut from the plot. Significant in that she was going to be the primary antagonist; the position was instead filled by Orochi.