Jöns Jacob Berzelius – seemingly known as JJ – was a Swedish chemist and is regarded as one of the fathers of modern chemistry. He carried out important work related to atomic weights and is responsible for the now ubiquitous notation of chemical elements (O for oxygen, H for hydrogen and so forth). He discovered or figured out how to isolate a variety of elements including selenium, silicon and thorium.
In Sweden, Berzelius Day is celebrated on 20 August in honour of his life. He previously appeared on a Swedish postage stamp in 1939. There is also a secret society at Yale University named in his honour (though the fact this is on the Wikipedia page suggests the society isn’t that secret…).