Unprofitable business, tokumei kumiai?

I (American, 5-year visa resident) have a friend (Japanese, citizen) who is starting a business. He isn't planning to take a salary any time soon, and the business isn't likely to be profitable in the short term. He's said that if he had more money, he'd like to expand the business's services. I'm a fan of his ideas, and I think his business would be good for our community. I'd like to help him. I believe that offering him money as a gift would damage our friendship. I'm thinking of asking him if he'd be open to a tokumei kumiai to give him the extra funds in exchange for a portion of ownership.

I plan to speak with a professional, but before I do so, can I get a reality check? I am not expecting to get my investment back. For financial planning, I'll be treating the money as gone the moment I hand it over. But, if the business does unexpectedly well, is it possible for a TK to pay my friend a livable salary before any disbursement comes to me? (Also, what sort of professional would be best to speak to? I don't want to waste anyone's time.)

by Tokyo_OhNo