Creepy Monotone: Magata's speech patterns. The real Dr. Magata, however has a suave voice completely unlike the decoy.
The Sociopath: Has shades of this. She seemed cold and remorseless after killing her parents, and had no qualms about killing her daughter or her lover/uncle (if indeed she did kill her daughter rather than the daughter committing suicide).
An Arm and a Leg: Done to hide her fingerprints and ensure that nobody finds out she really isn't Dr. Magata.
Armor-Piercing QuestionWho are you?When Moe asks her this, it's when she realizes that she doesn't want to kill her parents, and that's she's just a regular girl who wants out. Naturally, she got offed by Dr. Magata for her troubles.
Driven to Suicide: It's said in the penultimate episode that Michiru realized there was a certain freedom in death, but whether she killed herself or allowed her mother to kill her is left ambiguous. Moe thinks it was the former, but Saikawa says he doesn't know for sure.
Uncanny Family Resemblance: Michiru looks almost exactly like Dr. Magata when she was 15, and that's why Dr. Magata managed to fool the world into thinking that Michiru was her.
Walking Spoiler: This character's very existence is the main twist to the mystery of the show. No wonder all these tropes are whited out!