This is the 6956th most frequent Catalan word.
viktor
"Viktor" is not a Catalan word; it is a proper noun, typically a name.
This sentence uses 'Viktor' as a proper noun referring to a person.
Viktor és un bon amic meu.
Viktor is a good friend of mine.
Here, 'Viktor' is again a proper noun, but now it is part of a relative clause providing more information about the subject.
El llibre que Viktor va escriure és molt interessant.
The book that Viktor wrote is very interesting.
In this sentence, 'Viktor' is the subject of the action, liking to perform the activity described.