After Chibiki stops Akazawa from killing Kazami, it's mentioned that Kazami had killed 2 people. The second was a girl who was in Sakakibara's arms before he stabs her through the throat and asks Sakakibara if he remembers her (almost like randomly killing someone to see if they were the Extra). But, who was the first?
The two people that Kazami killed were apparently named KANEKI Kyouko and MATSUI Aki. Here's a picture of them together:
Right after the above shot, Kyouko (left) falls to the floor, apparently stabbed in the back by Kazami. He then chased after Aki (right), which is why she was running when she bumped into Sakakibara. Here, we see Sakakibara coming across Kyouko's body:
Shortly thereafter, Aki runs into Sakakibara's arms.
Then, Kazami stabs her in the neck, raising his kill count to 2.