As you quest with Caddoc and E'lara, you unlock various talents. These give you advantages in combat, increase health and mana, increase your gold pick-ups, and more. Review them all in the Compendium and pay attention to which talents are closer to unlocking so you can concentrate on those tasks to receive the bonus quicker. Many talents have multiple advancement levels; continue increasing those talents to become stronger and stronger.
Some combat talents, such as E'lara's Arrow Capacity and Arrow Damage, unlock by killing enemies. Even more unlock through combined kills, which encourages cooperative play with your partner. Others trigger when you find a certain amount of collectibles or secret areas.
Look to unlock your additional Health Vial and Mana Vial in Chapter 1. With a couple hundred combined kills, you close in on the extra Health and Mana Vials that will keep you alive longer in difficult battles. Once you top off at 270 combined kills, you increase the max health and mana of both characters. Each bump up in max health and mana gives you 50 health and 17 mana (until you reach the fifth upgrade, which gives you 60 health and 17 mana, and the sixth upgrade, which gives you 70 health and 17 mana). Another relatively easy talent to trigger is the extra Regenerative Vial; all you have to do is revive Caddoc or E'lara four times, which is going to happen in the course of your adventuring unless you're a master combatant. If you can find six secret areas, you unlock the extra inventory weapon slot, which gives you a huge advantage of carrying two weapons that you can switch between during battle.
Once you collect your second Dragon Tear, you unlock Battle Charge. Holding the Spell Activation button down will initiate a Battle Charge; you'll see your character light up with electrical energy, and you'll be temporarily charged to deal extra damage with your weapon. After you collect your seventh Dragon Tear, you unlock the more powerful Spell Link. If both players Battle Charge simultaneously (doesn't have to be at the same instant, but while the Battle Charge is still active), a Spell Link connects both players and its energy flies back and forth, seeking out enemies to damage.