Elon Musk is working hard for Tesla shareholders and the company's focus remains on delivering the Model 3, said Musk's brother and Tesla board member Kimbal Musk on Monday.

Kimbal Musk made the remarks during an appearance on CNBC's "Closing Bell." He said his brother is "doing a great job."

"We are trying to get great, happy customers," he said.

The comments come one day after Elon Musk responded to a Tesla customer on Twitter, who expressed frustration that Tesla had not delivered her Model 3.

"Sorry, we've gone from production hell to delivery logistics hell, but this problem is far more tractable," Musk said. "We're making rapid progress. Should be solved shortly."

Tesla has struggled with nearly chronic production and delivery delays for much of its history. The company has burned through billions of dollars but has not been able to make a steady profit. The Model 3, a mass market sedan, is key to the company's future.

Not only are some customers unhappy, shareholders are looking at a stock price that is down more than 22 percent over the past year. Tesla shares closed Monday at $294.84, down less than 1 percent.

Of late, concerns about Elon Musk's behavior has been weighing on the stock price. He has been intensely criticized at for both business decisions and behavior that investors have characterized as erratic and unprofessional.

On Monday, a British diver, who had helped rescue several young boys from a flooded cave in Thailand, filed a lawsuit in response to Elon Musk referring to the man on Twitter as a pedophile and "child rapist."

Elon Musk also was recently criticized for smoking marijuana on "The Joe Rogan Experience" podcast and referring to analyst inquiries on earnings calls as "boring, bonehead questions." He is active on Twitter, where he has made many controversial comments.

Lately, reports have circulated that the company is undertaking a search for an executive to help run Tesla.

Kimbal Musk co-owns a company called The Kitchen Restaurant Group, which operates three restaurant chains that emphasize farm-to-table cuisine at a range of price points. He also owns an urban gardening and farming company called Square Roots, and has a related foundation called Big Green, which teaches farming. He is on the board of Mexican food chain Chipotle.

Apart from his work in farming and restaurants, he is also close with his brother, and serves on the boards of Tesla and SpaceX. Kimbal invested in Tesla in 2008, during the financial crisis. He had been an investor in Elon's earlier company X.com, which later became PayPal, and co-founded an early company with his brother called Zip2.

His presence on Tesla's board, in particular, has been a bit of a sticking point for some investors, who have said Tesla's board is too close to Elon.

