Creating a video game is not considered "fair use" and it's not a gray area. The ITCH Terms of Service clearly state that you must own the rights to all the content you upload, and creating a drawing of another company's IP does not give you the right to commercialize or redistribute that content, because it is registered and protected by law.
That being said, with fan games, the following often happens:
- ITCH does not actively enforce the rule. In other words, if you upload content using a company's IP, ITCH won't say anything, but if that company files a complaint with ITCH, they will take down your game.
- Some companies are very protective of their content, but fan games usually have so little impact that they never even know they exist, or they're so obscure that it's not worth paying a lawyer to send a cease and desist letter. The problem here is that if your game gains notoriety, you could end up receiving a cease and desist notice or that the company could file a complaint with Itch. (For example, the Streets of Rage fan game).
- Some companies don't mind you creating fan games of their works or IPs, for example, Capcom. In these cases, as long as you're not making a profit, these companies won't ban or file a complaint about your game.