A Japanese term for a person who is initially very kind, loving, caring, and gentle to someone (or at least innocent) they really, truly like and care about a lot before their purely intense romantic, love, admiration and devotion becomes mentally destructive in nature, often through overprotectiveness, violence or brutality.
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 devoid of any life. They also tend to laugh maniacally, usually after committing a murder.
An interesting point to note is when Makoto dies or witnesses something traumatic, his eyes take the form of Yandere eyes too, possibly meaning that when the heroines go into yandere mode, they're essentially dead inside.
The Visual Novel
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 not making it clear that he had left her) and the second is the bullying by Nanami, Otome, and Otome's 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, but Kotonoha still is devoted to him but getting her calls blocked is very hurtful to her. What drives her over the edge is 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 deludedly "calls" him. When Makoto comes back to her, she might've been healed, but him getting killed by Sekai pushes her back over the edge, as seen in her showing Sekai Makoto's severed head before killing her, and cradling said head while sailing on her yacht.
She goes into yandere mode after witnessing Makoto and Sekai making out on the rooftop during the school festival and even the latter breaking things off with Sekai and apologizing does not change it. She attempts to kill Sekai, but Makoto intervenes, getting slashed by Kotonoha in the process.
The Visual Novel
Unlike Kotonoha and despite being emotionally frail, Sekai is far less prone to going yandere. The only circumstance under which she goes yandere is getting abandoned by Makoto while pregnant, after which she can either kill Makoto for abandoning her after she becomes pregnant or settle for killing Kotonoha, after concluding that she's manipulating Makoto. In the former case, she's only seen spending seconds under yandere mode, and when she kills Makoto, it's six 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, she makes a comparably rational decision to kill Makoto because of his pressure on her to abort their baby, although she immediately regrets her decision.
She kills Kotonoha in retaliation for trying to kill her and having wounded Makoto, then suggests to the latter that they should "begin with a clean slate".