Anime

Depends on how high quality you'd want it to be.

I'd probably just get some decent case, then look for a local print shop and see if they print stuff onto arbitrary objects.

I know someone who uses transparent cases, and just cuts out prints to put in between the case and the phone.

I don't really know what you are looking for specifically but here's a site that allows you to upload custom images for case customisation.

It's super difficult to find a non-toxic, non-chinese manufactured, high quality case, so for this search I discarded any of these attributes..

boomslank has some good stuff if you like the aesthetic and not a specific anime. etsy can be good if you're wanting something from a specific anime.

The slightly unfortunate thing is that the Pixel 6 is a bit old and in the world of cell phones if it's older than last gen then it's a relic. That by itself is going to make finding a case more difficult, but definitely not impossible.

The other unfortunate thing is that most cases online are going to be cheap chinese garbage. This is especially true on Amazon and eBay. Even some "officially licensed" stuff is pretty crap, as they just take their existing case and slap what amounts to little more than a sticker on it.

So basically you're going to want to find a quality case manufacturer and go directly through them. I found a case I liked on Amazon from SkinIt and have been using that for my Samsung phone. I would recommend going through their website though as it has a lot more options. They have some anime choices like DBZ, Cowboy Beebop, and My Hero, and you can even custom upload an image. This link is for the full silicone "pro" case but they also have a cheaper clear case option: https://www.skinit.com/collections/google-pixel-6-pro-case

Rhinoshield lets you customize a case and has cases for a few popular anime series in stock.