In addition to their basic attack, every hero has three different skills.
List of Skills
A passive skill that is always active and grants a variety of bonuses, depending on the hero.
A powerful skill that becomes available once enough Fury Points have filled the SP bar underneath your hero’s HP bar. Fury Points are gained by attacking enemies. Depending on the hero, the skill can be offensive or defensive in nature.
When you fuse Tomato Heroes to a hero, there is a chance that said hero’s Ultimate Skill will level up, increasing its power. If you match a Tomato Hero with a hero of the same star level, you will get a 100% chance to level up the Ultimate Skill. This probability decreases when you use Tomato Heroes whose star levels do not match the hero in question.
Fusing with certain heroes besides Tomato Heroes can also increase your hero's Ultimate Skill level. When selecting fusing cards, check the ULT Upgrade readout at the bottom of the screen, directly above "Main" to see the probability.
A skill that only becomes active when the hero in question is made the squad leader. It usually grants bonuses to the whole squad, and often times selecting the right hero for its leader skill can be the difference between victory and defeat.