Kei Tajima seems to be Mishio's best, if not only, friend. He takes her abuse with good humor and follows her on her strange detective missions. This leads Mishio to call him 'Watson' (after Dr. John Watson, Sherlock Holmes' friend and biographer) since she calls herself 'Sherlock'.
In Episode Five Kei is with Mishio when she sees the amnesiac Rin at a party. He goes with Mishio when she follows Rin to the restaurant and witnesses Ihika's rejection, and he is also at the scene of the crime with Mishio when they see Rin and Ihika getting attacked by Laura.