That's not how copyright works. The creator owns the creation and derivatives of. I know it's crazy common in the fan communities for fans to create and sell fanart and whatnot, but should the creators protest, they could lockdown the websites that sell fanart and/or sue the artists directly. Now, most won't because it's shit PR to sue your own fans, but the fact they don't enact the law doesn't change the law.
Consider Disney. Do you think someone off the street could open a T-shirt store and fill it with fan-shirts of Mickey Mouse, sell them for profit, and get away with it? No way in hell, Disney would be all over their ass for profiting off of their IP.
Now, the question is, will the creator of Aoi step in to do the same with Socio Sim? And Warner Bros for Fureddo? And Digimon, Sailor Moon, One Punch Man, and the rest? Who knows--but in reality, it's probably not likely. However, it still doesn't make Alex look any less shit.
For reference, quick definition of copyright, though it's far more complicated than that:
" the exclusive legal right, given to an originator or an assignee to print, publish, perform, film, or record literary, artistic, or musical material, and to authorize others to do the same."
Officially, anything is copyrighted the moment it's put down to paper. Your grocery list is copyrighted.
Socio Sim is a unique case though. Alex probably has clear claim to the name "Yandere Simulator;" beyond that, it'd tae a judge to decide. He can't claim Ayano's iconic design because she's a Unity model, and her original creator owns her. He can't claim the concepts of yanderes or murders or whatever else because tropes can't be copyrighted. He can claim the code he wrote--but who'd want to steal that from him? He can claim the dialog he wrote--but again, who'd wanna touch that shit? He can't claim he owns the original concept as that was made on 4-chan. He can probably claim the characters and their characterization, but their appearances probably not. So most of the stuff that is truly his is shit.
The Easter eggs he included are an interesting case because he might be able to argue he's using them for parody. However, that would require Alex admitting his game is a comedic spoof, not a super serious edgelord anime troperiffic deep plot game, and we all now how much it would hurt his damn pride to do so. So in that sense, he might even be able to get away with the Scooby-doo knock-offs by claiming they're parodies.
It also gets REALLY interesting considering how much of Socio Sim is actually commissioned work. All of the assets made by volunteers belong to the volunteers. However, it is possible for a creator to give/sell the copyright to the client (for example, a photographer could sell the rights of a photograph to the person in the photograph). It's also possible for the creator to allow the client to use the creation within limits. The Aoi model has those on her Unity page (for example, she can't be used for profit, which Alex has already breached.)
What gets really, really interesting about that is that those require contracts. And who wants to bet Alex and his volunteers don't have a legal contract to stand on? I know it came up before on this thread where someone wanted to make buttons for Socio Sim, and wanted a contract, but Alex wanted a polite "gentleman's agreement" instead which couldn't stand in legal water.
So it all depends on if the volunteers allowed Alex to have the assets they created full on lock, stock, and barrel, all rights included, or if the volunteers added disclaimers like "you can use these trees I made, but you must give me credit for them." Which means Alex, as not the creator, has to listen to the volunteers' stipulations, not use the assets all together if he won't, or risk being sued. It also highly depends on all parties having things in writing (which is likely as they seem to communicate through email a lot). Alex's sole defense also hinges on him specifically asking for and documenting that the volunteers signed away their rights to him. If he tries to pull a "but they volunteered for me~ I can do whatever I like with their works because they're just volunteers~ I can't give credit now because the game is incomplete~ wah~" he's up shit creek without a paddle, and any judge is gonna laugh his ass outta court.