With latest update in may 2020 the game received another addition to the class system "Alliances"

Similiar classes benefit each other or in some cases the whole team when you pair them together, a small symbol in the upper corner of the PVP or Mine fight screen will appear if there are boosts given due to a class alliance.

https://dungeoncrusher.fandom.com/wiki/Alliances

This spreadsheet currently only shows the basic heroes, but can be filtered for classes to find missing heroes you might have to add for a team set up

https://docs.google.com/spreadsheets/d/12YyMAuuJRNORhLkwhjCNBiuGj9G9w2xE/edit#gid=606984936

At the start of each round (League, Clan Mine, Castle) a check is done to see how many heroes you have of each alliance to determine buffs. All buffs remain til the end of the round, even if the heroes needed for the buff are killed early into the round.

If at the end of the round you lose buffs that increase a heroes HP all heroes on the playing field will lose this HP, any heroes reach 0 because of the lost HP will die before the new round starts.