Takes place in Dream Exorcist,it's a future chapter takes place after chapter 66 of Blue Exorcist.

Kotaro's POV

After everyone left the room,it was time to confront Mephisto about his true plans."I see now."I mutter uncrossing my arms. He grins raising a brow."See what?"he questions."You're afraid afraid of Nii-sama.That's why you had me come to this school and put a seal collar on me.So I could help you take him down."I start."Oh really?"he smirks."You know I'm closer to Lucifer' s ranks than you and a likely chance to beat him in a fight.You know I don't like being involved in sibling rivalry."I say."You're think so but that's not the case."he sighs."Then what.Why did you save me from the mercy of the council,stopping them from eliminating me?"I ask walking closer to his desk."You're my adorable little sister."he smiles.I slam my hands on the desk."Out with it!Rin,me,Shima,Flare,Igneel,Akumo,Damein,even Kurosaki,Izumo,Bon all of them you're using them as pawns in your game,why!?"I shout,my feet shaking from finding the truth."Sigh,you're starting to unravel it all.Yes they are my pawns but you're not.You're more like the king on a chessboard.I really can't afford to lose you."he explains looking down for the first.I grin thinking up my own plan to get the truth."How about this.I'll join Lucifer and be your second spy.You know he'll trust me.Besides,you can't trust Shima."I explain with the shadow of my hair covering my eyes.I watch his smile turn into a frown."How will I know you won't betray me?"he asked. "Well there's still the matter of this seal collar.Besides my class is falling apart because of Illuminati.Using them as test subjects is not OK."I protest."So you do care about them."he chuckles a little."Yes I do.They're my friends."I admit."Alright fine let's go with this plan."he sighs giving up."Don't worry,Nii-san."I say giving him a reassuring smile.After that I headed towards my classroom to tell my students.