I'm new to Itchio, and I couldn't find much help. 

Is there a general rule on what you can use in your game? 

For example, say I'm making a completely free game. Which of the following could I use?

A asset pack with unknown origin

A song that was on youtube, tagged as royalty free. Does that apply to Itchio as well?

A character from a movie or game. 

I've seen stuff that use copyrighted characters, music, etc. There seem to be plenty of Zelda, Mario, and Sonic fan games out there. 

Anyone have advice?

If @No Time To Play doesn't mind if I add a little more explanation. 

Legally you should not use content that you do not have the permissions to use, I say permissions, because you can buy or license content.

Fangames aren't really a gray area, you don't legally have the right to use someone else's content without their permission.

If the company or individual who owns the copyright wants, they can cancel your game and you have no right to claim, no judge will ever agree with you, because legally it is not a gray area.

In reality, what happens is that these games tend to have so little diffusion that the owners do not find out and never make a claim. Other companies do allow the use of their content in fangames.

itch.io has a reactive policy, they upload the game and if someone complains, they block it.

If you make a game with content that you don't know where it came from or whose copyright belongs to someone else, you run the risk of having it canceled.

How to minimize that risk?

If you use content from big companies, make sure it's from companies that don't have a problem with fangames, like Capcom.

If you use Nintendo or Sega content, and your game goes viral, it will most likely end up being canceled ("SOR remake" for example).

Try not to use content that you don't know its origin, for example, you could use some graphics that you found on the internet, you don't know that they were taken from another indie game and one day, here in itcho, someone realizes that your game uses the same sprite as another game, place the claim and your game will be canceled, again, you are taking unnecessary risks.

If you don't want to take risks, ask for help from an artist or use only royalty-free content and respect the license of each resource.

For example, at https://freesound.org/ you have a lot of effects and most of them are CC, or they simply ask you to place them in the credits.

Thanks! 

I've heard of how strict Nintendo is.

Oh, last thing. 

Any info on the Kaduki pack? Is he the actual creator, and is he ok with people using it? 

The place I got it from said yes, but his Twitter account and such are shut down. 

Sorry, her name is Katsuki Kiyoto. 

My mistake.

I couldn't tell you, I've googled the name, but I don't see anything like a video game resource pack.

That's okay. I researched it a bit myself and I'm pretty sure as long as I'm not reselling it or using it commercially I should be fine.