The Antibody that is always accompanying Akane, although you only get to fight him in the optional battles against her. Technically speaking, Kukuro is no more than a very enhanced version of a Doll Model HBL-S in a different color, but that doesn't make him any easier to defeat.
However, unlike Akane, who becomes more and more dangerous the further you battle her, Kukuro's difficulty stays at about the same level in all of the three battles in which you'll be facing him. Still, don't let your guard down, as by the third battle, his stats will be on par with Akane's, and if you're playing in Hard, they both will be hitting the HP cap and having an extreme amount of attack and defense. Despite all of this, Kukuro is still the best option you have for racking Beat Ups and Purging your Reyvateil in preparation for using the Ultra Supermoves and Flipsphere to take Akane down. Once Akane has fallen, Kukuro shouldn't be too much problem to defeat, although that most likely will mean you will have to repeat the same thing you did to defeat Akane. However, before you can even reach that point, you will have to be very careful, as like Akane, he can inflict all status ailments on the characters he hits with his attacks and also lower their defenses. Likewise, if he tries using his Energy Deduction Stored Attack, make sure to interrupt him unless you want the battle to get much longer and drawn out.