I was planning to make a game with Sonic characters and just sell it for free, and i searched the itch site for sonic there was a plethora of games, so am i allowed?

Your game is illegal, regardless of price. Although Itch is lax on this issue, they will be forced to take your game down should they receive a DMCA.

A lot of people seem to get away with it, but if your question is if it's legal to use copyrighted characters in your game without permission, I believe the answer is no.

https://www.aspectlg.com/posts/copyright-in-characters-what-can-i-use

You are not allowed, but most likely no one will care enough to action it.