It was revealed in Episode 18 that Chizuru is the one that killed Megumi. Megumi's father commented that Chizuru's perfume was in Megumi's room every night until she died, but it was hinted in episode one in Megumi's dreams that she killed her. She was also seen in the hallucination Megumi was having before she died.
Yoshie stated that Chizuru and Sunako have a special bond, like mother and daughter, with Chizuru playing the daughter. This is why Sunako is so lenient with her.
She prefers to drink the blood of young men, hence why she targeted Toshio and put him under her control. During her date with Toshio, she reveals that she had a husband who died in the war. During that same date, at the kagura festival, Toshio exposes her nature as a Shiki, and together with Megumi's father, impales and kills her. Toshio was able to resist her mind control because Natsuno bit him first and gave him different orders.