George Takei also posted this about him

My heart is heavy. Walter Koenig, our Ensign Pavel Chekov, passed away early today.

Walter and I met on the bridge of the Enterprise in the 1960s, and we stayed friends for nearly six decades. When Brad and I married in 2008, Walter stood beside us as our best man. Just last month, on Sept. 14, he turned 90, on the same day Brad and I celebrated our 18th wedding anniversary. I will always treasure that he and I shared that date. Walter was more than Chekov. He was a writer, a teacher, a thoughtful man, and a loyal friend. To the fans who loved him for so many years, thank you. He loved you too.

Rest well, my dear friend. Keep navigating a steady course. – George