I can’t list all the people who contributed to the design, it would be too many. I and Maxim Shafirov (the present CEO of JetBrains) did the bulk of the initial design back in 2010-11, and then there were very many ideas from various people. I lead all the language design efforts ever since, this does not include libraries. Roman Elizarov is leading the entire coroutines effort, he also suggested to add null-safety to Kotlin at the very early stage, even before joining the team. Ilya Ryzhenkov, Dmitry Jemerov and later Stanislav Erokhin participated in a lot of design discussions for 1.0. It’s many more people who were involved at various stages, and I’m afraid I’ve forgotten someone who made a big contribution. Apologies in advance.