Ai and Mina
Ai and Mina are two good friends, and apparently classmates of Makoto. Their friendship is put in danger when Ai suspects Mina is flirting with Makoto, who she has had a crush on for quite some time. Their argument is resolved by a game of Reaper Creeper. In Week Three, they are among the many possessed by the Red Skull Pins.
It is possible that Mina is a friend of Eri as well, as Eri is seen talking to a girl using her same image, but as the game tends to recycle images, it might just be another schoolgirl who remains unnamed.
 Another Day
In Another Day, Ai and Mina are optional opponents you can play in Tin Pin Slammer. In addition, they both appear to be friends of Shiki, and they are both members of Shiki's Prince fan group, the 'White Angels'.