There is lots of info on this around, a quick search should give you plenty to read.
It's his "own" Swedish carbon steel. The core steel is fairly simple carbon steel without alloys to give it wear resistance.
It is quite reactive, although some more than others, reportedly. It really depends where you live and the conditions (humidity, etc). Once you have developed a patina it will behave better but it will always need a watchful eye, I would never "store it" with out oil. And I wouldn't use it to cut onions... Although it does a great job at it.