the question arises when this game jam will end?
There is a countdown on the jam page (it also has the date and time in your local timezone) https://itch.io/jam/advjam2023
Does that mean he's already on his way?
Yes. The jam started 10 days ago. The rules on the jam page explain that you can use existing assets (such as character art), so long as you respect the licence the original author made those assets available under.
And the last question, is it possible to use someone else's character but with the indication of the original author?
