@miumiuw the core definition of an yandere is controlling, manipulative behavior and an unwillingness to let the person go to the point of violence. It typically is seen in psychopathic, sociopathic, narcissistic, or unstable characters---those who objectify people. Sometimes, the yandere is written as being capable of love (though, their jealousy typically comes across in the "murder other people" form; seldom does it comes across in the "kill my love" form. That type is seen in the more cray-cray versions. Not too the genre. Are we talking about a yandere in a strict romance or in a romantic comedy? It makes a difference. Likewise, characters can have elements of the yandere archtype without full-blown "yandere-ness". Characters usually work their way into it. It mixes in with the "my boyfriend is the devil" tropes. In the most simplistic of terms, yanderes hurt people they see as "romantic competition" for comedic effect or the romantic lead. If it's played off more seriously, as this chapter does, then it comes off as possessive, sinister, manipulative, narcissistic/sociopathic, and objectifying. Both siblings have "yandere" qualities, but Cedric is pulling the "cray-cray", low self-esteem/psychopathic high self-esteem that can't accept not getting what they want/desire. His brother pulls off the highly overprotective streak that includes "obsessive overkill" to anyone who they
might think is threatening them. Though, I don't think that
necessarily necessitates the "yandere" moniker.
As for Cedric---he has convinced himself/been convinced that his fiance betrayed him like all the rest, that she hurt an innocent girl, he's full on narcissism and the delusions imposed by the cray-cray "heroine". He cannot accept Ms. Protag going with
anyone else, particularly not Cedric's brother, whom Cedric has his entire life been compared to in lesser terms,
always the inferior, and never capable of mustering up until the Prince was deemed a "threat" and cast out. Basically, Cedric is the "spare". To him, not only has the woman he needed for support failed to offer it [as she herself recognizes], but he in turn has emphasized due to the reality warping of the "heroine". Cedric appears to believe that the protagonist has "cast Cedric aside" and he's lost to his brother. He wants to be the one to cast her aside, but at the same time, the non-mind warped part of his appears to still possess
some feelings, even merely jealousy, because effectively, he'll kill her before he lets her end up with his brother. And even then, he won't let her go. Um... if that doesn't
scream yandere, I don't know what does. =P