It's basically an origin series for the entire Batman universe. The closest thing the show has to a main character is Jim Gordon, but it is very much an ensemble cast.

Gotham is one of those shows that takes its time to get rolling Season 3 is where it starts to get really good. Not to say seasons 1 and 2 were bad they’re just not as good as the rest

In my opinion, the show is perfectly casted. Each actor plays their character very well, especially Ben McKenzie as Jim Gordon. You should seriously watch it.