A headmate is an "Alter" of someone who has Dissociative Identity Disorder. A headmate is their own person who has split off from the original person's mind, who has their own separate wants, goals, needs, gender, age, etc.
These headmates are usually formed as a coping mechanism, helping the person with DID to survive the traumatic experiences that they have been a victim to. A collection of headmates including the host is called a Multiple/Median System.
Due to the large amount of systems on Tumblr, it has been wrongly labelled as a "Tumblr trend" and been accused of mocking mental illness, despite it being a mental disorder in itself.
Someone who shares a physical body with one or more people, or someone in a plural system. This term was coined by non-traumagenic systems, though it can be used by any kind of plural system.
A headmate is also sometimes known as an alter (though mostly in traumagenic systems), or simply system members.
A women who serves the patriarchy; i.e. Cool Girl. Handmaidens either defend, support or claim to love pornography, pedophilia, rape, incest, BDSM, surrogacy, egg donation, transgenderism and prostitution etc. They often deflect arguments against feminists with "but but but... women to it tooooooo". Many handmaidens claim to be (liberal) feminists and will attempt to euphemize and appropriate women's subjugation by rebranding it "empowering" or "feminist", such as "feminist porn" or "sex work empowurrs me rawrz". Supporting the trans lobby, the porn industry is a handmaiden's cause.