Benrishi are authorized under Japanese law to practice patent law, but need not be lawyers. Generally, under U.S. ethical rules, lawyers may not be partners with non-lawyers. Instead, much like US patent agents, they need not sit for the general bar but must pass a patent registration examination. Unlike US patent agents, they may in some circumstances represent clients in patent matters in certain courts. Nonetheless, a New York ethics committee reasoned that, under some circumstances, US lawyers may form partnerships with Japanese Benrishi.
The opinion by the New York State Bar Association on Professional Ethics, Op. No. 1072 (2015), is here. The conditions include ensuring that the educational and ethical requirements are similar to other New York laws regulating partnerships with other foreign-licensed professionals.
On a related note, under some circumstances, of course, patent attorneys may form partnerships with patent agents, but that requires, like this, extreme care.