An LLC isn't required. You can partner with Altro as a US individual or as a US-An LLC isn't required, but you do need to be based in the US and legally able to earn income here, whether you partner with Altro as an individual or as a US-based business entity.
Either way, you're paid as a 1099 (non-employee) earner and are responsible for your own taxes on what you make. Some partners choose to form an LLC for their own reasons, such as liability separation or tax treatment, but that's a personal business decision. Since the right structure depends on your situation, we'd recommend talking it through with your own legal or tax advisor.