GenSane

"Because my 'nemi... is... the nicest... person... ever."

- Genya to Sanemi, Chapter 179

GenSane is the familyship between Genya Shinazugawa and Sanemi Shinazugawa from the Demon Slayer fandom.

Backstory
Sanemi and Genya are brothers from a family of nine, after their father was killed by a mysterious assaillant, the brothers promised to themselves and each other that they would protect their siblings and mother.

However, their promise proved to be short lived. Their mother didn't return home on time, worried, Sanemi went out to look for her. The mother returned before Sanemi, but she had been turned into a demon, killing five of her seven children in one blow. By the time Sanemi got back to their house, all of their siblings but Genya were killed. As the demon approached Genya, Sanemi tackled the "wolf" (that was actually their mother) through the window to save Genya, yelling for his little brother to run for it.

Terrified yet determined, Genya ran to get the doctor for their injured siblings, but when he got out, he saw Sanemi standing over the corpse of their mother. Unknowing that his mother was the demon, or "wolf", Genya screamed at Sanemi and labelled him as mother's murderer, an action that he reminisces regretfully.

Genya recounted all of this before what he assumed to be his "death" by an upper moon demon. At the end of the memory, he thought of Sanemi smiling after they've promised to protect their family.

Demon Slaying Corps
Sanemi joined the Demon Slaying Corps so that his brother could have a normal life without fear of demons, he used his marechi blood to intoxicate demons and kill them. He eventually became the wind pillar.

Genya joined the Demon Slaying Corps to reconcile his relationship with his brother and apologize for being so cruel to him after their mother's death. He makes it his goal to become a pillar so he can meet his brother again, which is an almost impossible task considering that Genya cannot use breaths.

The brothers meet for the first time in years at the Butterfly Estate. When Genya tries to apologize to Sanemi about what he said, Sanemi brushes him off and replies that Genya was annoying and that he had "no brother." It is said by Tanjiro, however, that Sanemi never hated Genya despite his coldness and that he had always loved him unconditionally. Sanemi was angry with Genya joining the Demon Slaying Corps as that would've made all his work and dedications to his job pointless- he joined so that Genya could have a normal, happy life, yet there he was; risking his life trying to apologize to Sanemi about something that he had already been long forgiven for.

Throughout the manga following this, Sanemi is hostile and openly states that he wants Genya to quit the Demon Slaying Corps. Genya persists and said that he wanted to help Sanemi, to become a pillar himself and fight by his side.

When Genya is severely injured and abou to be killed by Kokushibo, Sanemi intercepts the blade and fights the Upper Moon 1 demon by himself to protect Genya. Genya struggles to reattach his limbs, while he is doing so, he says that he can't let his aniki (big brother) die, not when they've come this far. Genya, along with Muichiro join Sanemi and Gyomei in their battle against Kokushibo. When Genya gets cut down by the upper demon and subsequently killed by the blow, Sanemi charges at Kokushibo yelling that he'll kill this "eight-eyed fucker" for killing his little brother, all the while so crying.

After Sanemi passes out from his injuries and exhaustion, Gyomei lays him next to Genya, when the latter calls for his brother. Upon seeing his brother's face, Genya is relieved that Sanemi is alive.

Cutting to Sanemi waking up, the first thing he sees is his brother's body starting to disintegrate like a demon's. He screams in anguish as Genya slowly crumbles, desperately trying to save his younger brother but to no avail. As Genya dies, he finally fulfills his goal of apologizing to Sanemi and being able to fight along his side. Genya apologizes for blaming him for their mother's death and for being a burden. Sanemi screams back that Genya was never a burden and begs him to not die. Saying that Genya couldn't die before him. Genya thanks Sanemi for looking out for him, to which Sanemi cries back how could be call this "looking for him" when Genya was the one dying. Both Sanemi and Genya are crying at this point, and Genya tells Sanemi that even though Sanemi tried so hard to protect him, he wanted to protect Sanemi as well.

Genya uses his dying breaths to tell Sanemi that he wants Sanemi to be happy, that he didn't want him to die, and to tell his older brother that he thought of him as the nicest (sweetest in some translations) person ever; and lastly, thanks 'nemi. Sanemi begs God to not take his younger brother away, screaming out in pain as the last of Genya disintegrates.

Quotes
"You should've gotten married, had some goddamn kids, and lived until you became a goddamned old geezer. For all the times we couldn't do a thing for mom, and for all the times we couldn't do a thing for our little siblings, you should've made your goddamn wife and kids happy as they would have been. Because I would've made sure that no demon would get near you..."

- Sanemi to Genya, Chapter 166 "Th... ank... you... 'ne.. mi."

- Genya's last words to Sanemi before disintegrating, Chapter 179

Fanon
GenSane initially started when they were revealed to be brothers in the manga, but the ship only started gaining popularity when their pasts were revealed and their motivations for joining the Demon Slayer Corps: eachother. Some fans ship GenSane as a romantic/platonic ship, but it is usually a bromance.

A semi-popular headcanon is that Sanemi's favorite food, ohagi, is a dish that Sanemi doesn't know how to make himself, but Genya does.

As Genya is now dead in the canonverse, fanworks will likely take place in an AU where he survives or prior his death.

Fandom
FAN FICTION
 * Genya/Sanemi on FanFiction.Net
 * Genya/Sanemi on FanFiction.Net

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Trivia

 * Despite being five years younger, Genya (5'11) is taller than Sanemi (5'10) by a little.
 * Sanemi's favorite food is ohagi (red bean cake) and Genya's is watermelon.