Heix

"Toby fox doesn't like AU's!! All of them mischaracterise characters!!!! Especially Sans!!!!"

The headcanon of Heizou still keeping the green pebble him and his friend found back then is rather special once you think about it: the trait Heizou dislikes most in a human being is dishonesty, and the trait he loves most in a human being is, well, honesty. All of the gifts his friend gave him were stolen and covered up with lies his friend made to cover his background: except for the green pebble. It was something him and his friend found together, scared and it eventually became a symbol of friendship between them. It was the only gift his friend gave him without lying about it, and Heizou keeps because of it.

Heizou's first constellation is named after The Kindaichi Case Files, a detective Japanese novel (also has a manga and anime adaptation as far as I know), and the first two pages talk about a young man trying to rid himself of guilt by helping others (in this case, helping a girl who threatened to commit suicide) after hurting (by slapping) one of his students who was caught by the police in an act of prostitution and she later on fell into coma and died three days later from brain haemorrhage (she also had a heart disease) and despite it not having any connection to him slapping her, he was still devastated, feeling all the accusing looks of parents and students on him, began to drown his sorrows by drinking every night because of it, and once a girl called him and threatened to commit suicide, he decided it would be a great opportunity to start his life anew.

Heizou holding his friend in his arms for the last time and choosing to run away from his home, to live by himself and commit himself to the goal of making criminals fear his name and think before taking action (all tied up to his friend) vs Ei holding her sister in her arms for the last time and choosing to lock herself in the plane of euthymia, unable to lead a nation like her sister could and dropping the responsibility of it on her puppet.