“Menhera” (メンヘラ) is a Japanese internet term for “mentally ill people” that left its target audience and, considering the environment, became a derogatory word that stigmatizes the mentally ill and gets carelessly thrown around when someone, often a troll, doesn't like someone.

On 2channel's “Mental Health” (/utu/) board, some users noticed that “mental health” (メンタルヘルス) was shortened to “menheru” (メンヘル) and attached the plural “-rā” (ラー)¹⁾ to get “menherā” (メンヘラー).²⁾ Back then, “menherā” originally meant “mentally ill person” and it was properly used as a demonym by people who actually considered themselves to be mentally ill³⁾ until the definition got lost.

The chōonpu (ー) would later be dropped, creating “menhera” (メンヘラ), and spread to other parts of 2channel where it quickly became subjected to the euphemism treadmill. As a result, “menhera” was turned into a derogatory word for mentally ill people since it was seen as a substitute for “kichigai” (キチガイ), a harsher insult that meant “lunatic” or “madman”.

Around 2010, the definition of “menhera” began to overlap with “yandere”⁴⁾ which would cause widespread misunderstandings about what the two terms actually mean, aside from a vague connotation with mental illnesses. It eventually became a point of contention, so the definitions of “menhera” and “yandere” changed with the following copypasta (formatted as a chart on this wiki) setting an example.

Of course, this wasn't a perfect solution as it just moved the negative traits of a “yandere” (e.g. aggression, self-centerness) over to “menhera”, replacing the yandere's mental illness with a blind devotion, then you have people who suggest that “yandere” is fiction and “menhera” is real-life. To this date, there is still no clear consensus on what a “yandere” is supposed to be.

• Both terms shouldn't be confused with hypersexuality or narcissistic personality disorder.

• Menhera on Pixiv Encyclopedia
• Menhera on Niconico Encyclopedia