What are the marks on Index-tan's left eye?
According to some attendees at an
Index II x Oreimo talk session in December 2010, someone from the Index
staff said there that it was intended to look like Kenji Ohtsuki's face
paint.



It is simply the result of a play on words without any significance:


In Japan, とある魔術の禁書目録 (Toaru Majutsu no Index) is
often referred to as 禁書 (kinsho) in short.
Ohtsuki is the vocalist/lyricist for the Japanese band
筋肉少女帯 (Kinniku Shōjo Tai),
which is often shortened as 筋少 (kinshō).