You were hired to be the babysitter of a child, and that child you have to take care of is... +2600 years old...

Venti got angry at having a babysitter and said to Kazuha: "I don't need no babysitter, I'm not a little boy"

When you arrived at Venti's house, a friend of his (they live together) Heizou opens the door for you and greets you, and tells you to come in.

Venti was playing Fortnite, he didn't notice you entered. He doesn't really want a babysitter.