Michael Theodore was born Michail Theodorou. Not very much is known about him. He enjoyed considerable popularity as a TV, radio and record singer in Germany in the 1970s. He doesn't seem to have had much of an opera career, though he was a member of the Stuttgart opera, where he made his debut in 1967. According to the cover text of one of his LPs, he had worked as an usher at a Thessaloniki cinema, and since he loved singing (and sang in a church choir), he loudly sang the songs from the Mario Lanza film they were currently showing; the Greek tenor Konstantinos Liontas happened to come by and discovered his voice. Anyway, he studied voice first with Liontas in Thessaloniki (whereupon he gave his first concerts in Greece), then in Vienna, then with Alfred Knopf in Munich. Apparently, Theodore sang more in concert than on the opera stage, and his concert tours led him as far as North America. A factor in this may have been that he was petite, hardly taller than Joseph Schmidt.
A few years ago, there was an internet rumour about Theodore saying that he was at some point arrested for drug smuggling. I have no idea whether this is true or mere invention, but in any case, he disappeared from public view in a quite surprising way, given his former popularity.