Elemental Hero Electrum is correct.
Reason being, in addition to being LIGHT he is treated as being EARTH, FIRE, WIND, and WATER. Those of us who remember "Captain Planet" will recall that they used their rings to summon him. The rings were EARTH, FIRE, WIND, WATER and HEART. Hence the first clue "Where's the heart?"
That also tied in the next clue of "combining efforts" and "who to ATTRIBUTE" the answer to. Maruno's answer of Homunculus the Alchemic Being has a look very similar to Electrum.
Nicely done Maruno. You're up.