Emiko Takamiyama

Futo-oyama Buko is the leader of a girl's gang. She is the only person Yoko is scared of. Her father is the chief of police in the city, making her the natural rival of Yoko, whose father is the head of the Yakuza.

In episode 21 Yoko eventually figures it must be her who is "ET". This leads Ranze to question if her initials should be "BF" (using the english order Buko Futo-oyama, not FB) but Yoko clarifies that this is only her nickname.

Her real name is Emiko Takamiyama (ET by English order, though since the Japanese say Takamiyama Emiko this should really be TE)

(Source: Tokimeki Tonight Wiki)