Isaac

Context
Isaac is the character Not Sus plays as in The Binding Of Isaac Repentance Logic.

Personality
Isaac is just a player character and has no actual personality.

The Forgotten
The Forgotten is Terra Marc’s player character, so they are obviously friends

Duke Of Flies
the Duke Of Flies is the first boss Isaac fights. The both have extreme hatred for each other.

Monstro
Monstro is the second boss Isaac fights, making them again quite obviously mortal enemies

Delirium
Delirium is the final boss of Afterbirth+ and was fought early by Isaac and The Forgotten because of a bug caused by Glitch. Delirium is the first boss Isaac loses to due to him and The Forgotten being unprepared for the battle.