Grateful set works like this:
A hero gets a stat similar to how eliminate or guardian works adding onto the percentage in the battle.
For instance a hero gets 25% attack 25% defence or no benefit at the fight screen (hidden 25% hp) which is still active.
Each buff is hero specific. One hero can get attack the other hero can get defence therefore stats on each hero can change for each fight.
Here are ss to show stats changing per hero: