Generation Xerox: Later in the game, we find out the Miki may be becoming mentally ill/delusional due to guilt over her aunt's death, just like her aunt went mad from the guilt of losing her baby. In the bad end, she dies too (though it's not suicide). It helps that Miki has, according to her family, an Uncanny Family Resemblance to her aunt.
Gory Discretion Shot: Whenever Miki gets caught by the monster, or if she gets killed off in the bad ending.
Unreliable Narrator: Madness runs in the family and Miki is no exception. The possibility that everything is a hallucination is left in the open, even in the good ending which requires you to insist it's real.
Dogged Nice Guy: Has an obvious crush on Miki and goes out of his way to help her with things he has no business helping her with (carrying all her luggage despite having an injured arm, for one thing.) It reaches kind of creepy levels when he has a tendency to barge into her house when she doesn't immediately come to the door.
Driven to Suicide: She slowly goes insane after her miscarriage, and with no one giving her emotional support, she eventually gives up on life and hangs herself.
Love Redeems: If the player chooses to believe the monster is real, her reconciliation with Miki helps her to move on and leave the house.
Posthumous Character: She's been dead three years at the start of the story, and even though she's the monster haunting the house everything you learn about her you learn via flashback and notes lying around.
Tragic Monster: She is the creature haunting the house and following Miki every night. It's implied she doesn't even realize she'll kill Miki; all she wants is to make up with her niece.
Jerk Ass: While it can partially be attributed to having some of the mental health issues that seem to plague the family, the woman's just mean sometimes. Such as making Miki live in her aunt's house because Miki's been diagnosed with depression and her mother doesn't want to be around a "crazy". Or laughing at Saeki when Saeki has a miscarriage.
Kick the Dog: Makes Miki live in Saeki's old house because she wants nothing to do with her depressed daughter. Laughs at Saeki for losing her baby.
You Are What You Hate: Despises her own sister and her daughter for being mentally ill; Saeki went mad from grief and Miki suffers depression. She just happens to be suffering from some kind of mental illness herself.