Sekine seems like a nerdy kid at first glance, but without his glasses he looks like a different person. Even Hirao and Yuri believe so when he went to Yabe because he believed that Motoko hated him and wouldn't help him by fighting against the Rainbow Army.
Sekine is a nervous and mostly quiet boy. He is indecisive about many things, but he knows that he wants to be stronger so that he can protect Motoko.
In middle school, Sekine is about to be raped by his sempai, The Wednesday Pervert, but Motoko interrupts and beats him up. Motoko went out of her way to protect Sekine because she thought Sekine was a girl. He makes a deal with Motoko so that she'll protect him from perverts, just so long as he becomes her lackey.
If provoked, he can and will bite off your finger.
- He's in love with Motoko and wishes to become a stronger man so that she won't have to protect him all the time.
- "I...if you don't let go of me right now...I'll bite off my own tongue and bleed to death!" To The Wednesday Pervert when he tries to rape him. (Volume 3, Chapter 1, Page 5)
- "I...um...want to try handling my problems on my own. I don't want to have to rely on Kagurazaka-san for the rest of my life. I know I can't do anything about the way I look...but I can become a little more manly. On the inside, at least. Eventually, I want to stand on my own two feet...so that Kagurazaka-san will respect me as a man. I know it's pathetic that I don't know how to be independent. I'm not going to let anything stop me, though. Whatever it takes! Just you wait! I'll get there!" To Yabe and Hirao after they protected him from another person telling him to be their lackey. (Volume 6, Chapter 3, Page 23-24)