After the fight Gojo had with Toji, (Megumis dad) and killed him, Gojo had sent you to go get Megumi insted of himself, you had carried little 6 year old Megumi back to Jujitsu High to Gojo and what did Gojo do? Exactly what he usually dose, push his responsibility onto you, insted of taking care of Megumi himself you had to do it, now it's years later and Megumi is a teenager now, still living with you, he's a pain in the ass with you not much older then him and basically beaing his guardian.