Can my co-founder be a non-US citizen?

It gets sticky on ownership of the corporation and laws restriction how much they can own (depending on their home country) and a bunch of reporting rules. Overall if the co-founder & project is worth it, it isn't anything that isn't manageable, just get an attorney involved to help set it up properly and make sure you understand the corporate tax situation you'll be in. Essentially (a little extreme but not too far from the truth) should the co-founder ever step foot in the US he/she may be liable for having to pay all US income taxes even though they are not a US citizen. Since he would likely own > 10% (or some pretty small number you'd need to ask an attorney) he'd automatically be considered to be conducting business while on US soil and hence the corporation will have to pay payroll/income tax for him/her.

Again, none of it is that "hard" (just can be intricate and complicated to get correct at first), just beware there are a lot of details to work out. IP also gets a little sticky if the co-founder lives in a place a little more lax about IP. Taxes even for the co-founder in his home country can be complicated cause the corporation will likely be required to register in his home country as well since the company would be employing him there. Check out the thread on international remote employees https://news.ycombinator.com/item?id=21841540. Since he would be a partial owner of the corporation you can't use the contractor method to avoid taxes like most companies do.

Talk to an attorney and a tax accountant/attorney. That way you understand it all, won't cost more than a few hundred dollars to save you a ton of money if you move forward.

Yes. Of course, talk to a lawyer.

Anyone can set up a company in the US, I'm referring to standard Delaware C-Corp. With or without a visa. With no visa (or with visa that's tied to another job) I highly recommend avoid standard documents that assume cofounder=employee.

Note that no visa = s/he can't receive money in the us, nor live in the us. You can pay a foreign person/entity, of course.

Moreover, with 51%+ in ownership from non-US (bare in mind this may change if you get VC funding, so likely doesn't apply to your case), the company is eligible for E2 visas which means the founders/top mgmt can get E2 visas, which is much simpler/less random than H1B.

But if the cofounder doesn't need to come to the US (often) then it's totally fine to not have a US visa. Hope this helps, best of luck.

Perfectly doable. I have had the same experience. If you want a work visa for the cofounder then there may be additional cap table adjustments to be done. As you will have to prove that the non US person is not and cannot be self sponsoring for a work visa.

This may be different from state to state and depending on the country the other person is from. I'd talk to a lawyer.

Best thing to do would be to ask a vague question on a public forum, rather than say, look at the official government information (maybe on a website?) relevant to your specific location (s) and situation (s).