In Nobara's case, Raiko's the only one she's very close with, and she doesn't even spend much time with Runa and Rie. She's also shown hugging Raiko after Momoko's apparent death, and hugging is more of an intimate act in Japan.
As for Mika, she was the first person to actively make an effort to befriend Kamen after the Halloween party. Kamen doesn't have many really close friends among the survivors apart from her complicated relationship with Raiko, and Mika doesn't have any friends apart from her obsession with scaring Raiko, so it's possible Mika is romantically interested in Kamen.
As for both of them, if this theory is true, then it will mean that every cast member is involved in the Love Dodecahedron.
It might explain why she's so obsessed with scaring her, although she's a relatively good sport when Kamen kisses Raiko, being more intrigued at what finally gets a reaction out of Raiko than jealous.
The way Raiko talks about her sister's death, it happened rather suddenly during Raiko's birthday trip, and was apparently the result of Raiko failing to listen to Reiko's side of the story about some incident. Raiko specifically mentions that she knows to consider the worst-case scenario, and "I had learned that the hard way, and knew all too well the consequences of misinterpreting a situation like this. Or putting it off as a joke" after Momoko's fake suicide, implying that Raiko didn't realize her sister was suicidal until it was too late.
It's anyone's guess as to what would have traumatized Reiko enough to cause her to be Driven to Suicide, or why Raiko would flippantly say she'd rather commit suicide rather than wear her mother's cat costume, but suicide seems like the most likely cause for Reiko's death.
It's the only way to avoid one of the survivors of the party falling prey to Sudden Sequel Heel Syndrome. Besides, none of the characters seem like the type to kill anyone.
Considering that he's dead in one version of the ending, the creators may want to avoid having to specify which one is canon. Even if he lives, he may still be in the hospital, and if even Taiko finds his stalking Momoko unforgivable, it's unlikely that anyone else wants him around.
