I know Yuri Gagarin was the first Soviet, and man, in space. Who was the first Russian (not Soviet) in space?
It is depending on your definition of a Russian.
Gagarin was born in the Russian Soviet Federative Socialist Republic, so he was the first Russian in space.
Sergei Krikaljow (also a Russian) started as a citizen of the Soviet Union, when the Soviet Union was dissolved on December 26, 1991 he was in space. So he was the first citizen of the Russian Federation in space.
The first manned Soyuz after the end of the Soviet Union started on 17 March 1992 (Soyuz TM-14) with Alexander Viktorenko and Alexander Kaleri. It was Alexander Viktorenko third start, so he was also a Soviet Cosmonaut before. But it was Alexander Kaleri first spaceflight, so he may be the first 'pure' Russian cosmonaut.
I haven't checked for Russian cosmonauts, which were not trained during Soviet era.
Gherman Titov is up to now the youngest Cosmonaut (started at age of 25 in 1961). The Soviet Union ended 1991. That's now 25 years ago. So there is up to now no Russian astronaut who was not a citizen of the Soviet Union before.
Perhaps there is also an American astronaut with Russian origin?
If you are talking about ethnicity, then the answer is Gagarin, who was an ethnic Russian.