By playing during the opening hours of Nexomon: Extinction, you will start earning hearts. They can often be obtained by completing quests, or if you're extremely lucky you may be able to loot some while you're exploring the world.
Cores are actually a very important part of the game and can be used to give your Nexomon an edge in different areas. Hearts will provide a stat benefit to any Nexomon you equip them on.
To equip the cores of a Nexomon, open the Team menu and click on the Nexomon you want to upgrade. On the right side of the screen, you will see two diamonds, one for skills and one for cores. Click on the hearts option to access the Hearts screen.
You can equip up to four different cores on a Nexomon at a time. Click on any of the open slots and you'll get a list of your available hearts, and hovering over each one will give you the details of the perks it offers.
You can also synthesize the cores of the shards that you find throughout the game by smashing rocks with multi-colored gems. If you bring these shards to Parum's Synthesis Building, you will be able to create different types of hearts, at different levels of efficiency. The higher the level, the more boost your Nexomon will receive from the Core.
You should normally try to build on the strength of a Nexomon with cores, rather than trying to cover up its flaws. The Nexomon tanker should get hearts of health, while strong hitters will deal even more damage if you increase their strength in combat.
