There was very little on TV Tropes but I know it happens with a good bit more regularity than suggested there. Cassandra was a figure from mythology who was cursed by the gods to be able to see disasters that were coming but at the same time find herself unable to make anyone believe her. This has since been a popular trope. What I'm looking for then is someone who realizes something very important and serious and tries to tell people or warn them, but absolutely no one believes them. Bonus if they get mad or think that (s)he's deliberately lying for personal reasons, the more antagonistic the better.
One of the most common setups seem to be something like one person suspecting a particular person of not being what he claims, and eventually finding proof that the person is really a villain seeking to harm their friends. The person desperately tries to convince the others, but they dismiss it, possibly chalking it up to paranoia, an overactive imagination or attention seeking in the case of a less mature person, or even some sort of grudge. In the last case in particular, the Cassandra is often reproached, possibly driving a wedge between them and their friends, because they think the Cassandra's trying to hurt someone they like and trust. Worst cases have this villain character be someone's crush, particularly the crush of the Cassandra's crush, adding in suggestions that the person's just plotting to break the two apart.
Of course, that's not always the case. There are many kinds of "terrible truths" the Cassandra could know. Perhaps they can see entities that others can't. Perhaps they know about an event or plot soon to happen. There's all sorts of possibilities. The core though is that the Cassandra knows the truth and struggles with people who are supposed to trust and believe them but instead dismiss their warnings outright.
One of the most common setups seem to be something like one person suspecting a particular person of not being what he claims, and eventually finding proof that the person is really a villain seeking to harm their friends. The person desperately tries to convince the others, but they dismiss it, possibly chalking it up to paranoia, an overactive imagination or attention seeking in the case of a less mature person, or even some sort of grudge. In the last case in particular, the Cassandra is often reproached, possibly driving a wedge between them and their friends, because they think the Cassandra's trying to hurt someone they like and trust. Worst cases have this villain character be someone's crush, particularly the crush of the Cassandra's crush, adding in suggestions that the person's just plotting to break the two apart.
Of course, that's not always the case. There are many kinds of "terrible truths" the Cassandra could know. Perhaps they can see entities that others can't. Perhaps they know about an event or plot soon to happen. There's all sorts of possibilities. The core though is that the Cassandra knows the truth and struggles with people who are supposed to trust and believe them but instead dismiss their warnings outright.
