Originally used as a tag for human powered vocaloids of the Z-kai CMs, the Benkyo Shiroido is the nickname given to the 4 main characters used in Z-kai MADs.

These characters are as follows (nickname | real name):

Ikemen (イケメン) | Kamei Yuuma (亀井有馬) His nickname's literal translation is "handsome guy"

Yuji (ユージ) | real name: Okano Kenzo (岡野謙三) His nickname is based on model Thomas Yuji Gordon (トーマス・ユージ・ゴードン) who is similar in appearance

Sock Lee (ソック・リー) | real name: Kato Ryo (加藤諒) His nickname is a play on the NARUTO character Rock Lee (ロック・リー) who is similar in appearance

Megane (メガネ) | real name: Tamekuchi Yutaro (溜口佑太朗) His nickname's literal translation is "glasses"