Actress Again's Boss Rush mode, where you face EVERY boss from the previous games, has Neco Arc (that cat thing) describe that Kohaku's (the one in the hood) lab has a Stargate now and is named the Great Cat Coliseum.
Shiki (Tohno) being the only one with a real storyline to it (you just randomly get Roa, Shiki Ryougi, Neco Arc, or Neco Arc Chaos as the host with the others), goes to the Coliseum from Kohaku in Magical Amber mode (when she's wearing the hood) pretending she's been kidnapped.
Leading to it to being some elaborate ploy for Shiki to recognize Akiha as a woman.
The reason why Kohaku makes the Makikyuu X this time, is because Akiha is inferior to the other girls' in a certain area. Akiha even asks how huge her chest is now.....with Shiki saying in a sneaky way they've only grown in size, not proportion.
As Actress Again is supposed to be the last MB, she even gets a new sprite.
The storyline version of the Actress Again fight is here
And the cutscene before it here
Speaking of huge only in size, I remember a doujin where only her breasts grew as successful but they stayed the same shape, making her look like she has a iron board under her shirt. Can't remember where to find it again though.