Class Skills can be upgraded by using them. You can see how far you are to fully upgrading a class skill by accessing the class skill section upon load-out. Fully upgraded class skills usually allow the unit to gain a better class skill, and can also allow other units to use that class skill. For example, fully upgrading Yarida's class skill Two Spears unlocks Three Spears for Yarida, and Piekron also gets the ability Two Spears. In some units, when you upgrade their last class skill, that unit gets a special bonus which is a boost to attack power, status effects rate, depending.