Kettler GmbH was founded in 1949 by Heinz Kettler. The company initially manufactured patio furniture, but went on to make a huge range of leisure and sporting products. As an aside, Kettler were based in Parsit in the Eastern Ruhr, about 10 miles downstream from the Möhne Dam that was famously destroyed by the Dambusters. Much of the adjoining town of Neheim was destroyed by a 10m high Tsunami when the dam broke. Every British person of a certain age will have seen the 1955 film and will not readily forget the sequence with the car headlights disappearing beneath the raging torrent.
In 1977 Heinz Kettler developed an aluminium bicycle frame, and in 1978 launched the Kettler Alu Rad (Kettler Aluminium Bicycle). I cheekily suspected that the company had developed a certain, utilitarian, 'expertise' in manipulating aluminium tubing to make its patio furniture and applied this to bike frames.
As I remember it from a bike show at the time, Kettler made a lot of noise about being the first in the world to make a bicycle from super-light aluminium. I found this slightly odd, as firstly, Caminade of France manufactured an aluminium frame in the 1930s and ALAN of Italy had started producing their aluminium frames in 1972. Secondly the Kettler Alu Rad was anything but super lightweight. It was a rather solid, reliable, chunky machine, slightly reminiscent of aluminium patio furniture - more suited to merrily trundling down to the local shops than winning the Tour de France.
The Kettler company went bust in 2015, saved itself by selling the Kettler bicycle business to ZEG, a German buying group, and then went bust again in 2018. However, Kettler branded bikes are still available today from ZEG.
In terms of derailleurs, Kettler rebranded Sachs-Huret derailleurs during the 1990s.