Headcannons may contradict eachother.

Blue is best friends with Error.

Blue is friends with the bad guys.

Blue is secretly depressed because he doesn't like being looked down upon and called childish, only Error knows about it.

Blue and Error know all of eachothers secrets.

Blue secretely helps all of the Bad Giys problems that they have and helps them feel better about themselves.

Stretch is abusive towards Blue.

Blue is hated in his Au but nobody but Error knows that.

Dust and Blue used to have a relationship until Dust cheated on Blue with Horror, Blue's relationship with Dust and Horror is still bad.

Blue is a Yandere, but not the killing Yandere, if anyone hurts his friends and family, well let's just say that things won't go well with them when Blue finds out...

Blue knows about the balance.
Ink&Dream do not.

Blue hates Ink because he's unknowingly hurting Error and ignorant of it.

Blue likes Dream because he's Nightmare's brother.

Killer can feel emotions.

Nightmare can turn Passive if he feels he is in a safe environment.

Blue is friends with Lust because he feels bad that he gets called a slut for something he doesn't do, it was Ink's fault Lustale was created,not Lust's.

Dream misses Nightmare and wishes for things to be normal.

Nightmare misses Dream and wishes the balance wasn't a thing so everything can be normal.

Dream and Ink have a tense relationship because of Ink being emotionless.

The bad Guys treat eachother as a tight knit family.

Blue and Error are aware of who Fatal Sans is.

In return for Blue helping the gang, the gang also helps Blue when he's feeling very insecure about himself and thinks about his abusive Au.

Blue is the most mischievous being in the Multiverse.

Not in order so that people don't just go to their favorite character and just read that part and so if it updates you can just scroll down to it and not have to guess which character gets a new headcannon.