After rewatching Better Call Saul, I got the feeling that Mike Ehrmantraut is the primary protagonist in Season 2, while Jimmy McGil/ Saul Goodman is the deuteragonist/secondary protagonist. It seemed like Mike was given a bit more screen time than Jimmy. While Jimmy/ Saul is the main character of the show, I believe Mike is the main character in Season 2.
If I were to break down the importance of the characters by season, I would say it would look like this:
Season 1: Jimmy/Saul > Mike > Kim > Nacho.
Season 2: Mike > Jimmy/Saul > Kim > Nacho.
Season 3: Mike and Jimmy/Saul (co-protagonists) > Nacho > Kim > Gus.
Season 4: Mike and Jimmy/Saul (co-protagonists) > Nacho and Kim (co-deuteragonists) > Gus.
Season 5: Jimmy/Saul > Mike > Nacho and Kim (co-tritagonists) > Gus.
Season 6: Jimmy/Saul > Kim > Mike > Gus > Nacho.
Overall?
Jimmy/Saul > Mike > Kim > Nacho > Gus.