There are 5 different classes in Legacy of the Beast: Warrior, Sentinel, Magus, Gunner and Assassin.

With Affinity Advantage, combat between the various classes is similar to rock, paper, scissors. If a class has an advantage over another class, they not only deal more damage, they also receive less damage when attacked from that class.

Warriors have the advantage over Sentinels, Sentinels have the advantage over Magus, and Magus have the advantage over Warriors. The other 2 classes, Assassins and Gunners, have the advantage over each other. If there is no affinity advantage, the classes are considered neutral.

Affinity is indicated in battle by the colour underneath the target enemy. Green means your character has the advantage, red means the selected enemy character has the advantage, and yellow is neutral.

If you're having a hard time beating a boss, note their class and see if you can make up a team of the class that has the advantage over the boss's class to do more damage and receive less damage in turn.