The main heroines can go into yandere mode if they are heavily hurt emotionally, abandoned or treated very poorly by Makoto and always leads to a bad ending. Whenever a character goes into yandere mode they lose all sense of right and wrong and don't care about the repercussions of their actions, with only their goal in sight. Characters in yandere mode are identifiable by their eyes, which are colored heavily black and void of any life. They also tend to laugh maniacally, usually after committing a murder.
An interesting point to note is when Makoto dies his eyes take the form of Yandere eyes too, this possibly means when the heroines go into yandere mode they're dead inside with all life gone from them.
The Visual NovelEdit
Kotonoha is very prone to going yandere when she doesn't accept Makoto leaving her. She often makes many attempts to win him back and if she's not successful she can either kill Sekai or kill herself. There are two main factors that lead her to go into yandere mode, the first is Makoto leaving her after having sex with her best friend (moreover, Makoto did not make it clear that he had left her) and the second is the bullying by Nanami, Otome and her friends, which can drive her into yandere state even if the worst Makoto did was ignore her without having sex with her. Once she crosses the threshold for yandere she doesn't come back and is in the same state for days even at home.
She's at first ignored by Makoto after dating for a while because he finds her too tiring and lack of sexual activity but Kotonoha still is devoted to him but getting her calls blocked is very hurtful to her. What drives her over the edge is when Setsuna sabotaging her attempts,getting raped by Taisuke, Otome and the trio's bullying and finally Makoto telling her that he didn't love her anymore. She starts to pretend Makoto's her boyfriend and spend fake phone calls on him. When Makoto comes back to her she might've been healed but him getting killed by Sekai only endangers her mental state to the point of insanity as she sails on a boat hugging Makoto's severed head.
The Visual NovelEditUnlike Kotonoha and despite being emotionally frail she's far less prone to going yandere, in fact the only way she goes into yandere mode is getting abandoned by Makoto whilst pregnant. When she does however go into yandere mode she can either kill Makoto for abandoning her after getting her pregnant or settle for killing Kotonoha, after figuring out she's manipulating Makoto. In the former case, she's only seen spending a seconds under yandere mode, and when she kills Makoto it's 6 months after going yandere and she's no longer in that mode, and does it in a way she doesn't get caught, since her whereabouts are unknown years after the incident.
She doesn't go into yandere mode, rather makes a rational decision to kill Makoto because of his decision on aborting their baby, although she immediately regrets her decision.
She kills Kotonoha in retaliation for unintentionally trying to kill Makoto, then suggested to the latter that they should "begin with a clean slate" before presumably kills Makoto.