EXP Eggs are a special type of resource, but categorized as hero. It has some similarity to Heroes, but does not function like one. Their only purpose is to be fused with other heroes to increase their EXP.
EXP Eggs can be level up just like any other heroes. They have the same experience level progression like a hero does. Leveling up an EXP Egg is just like storing EXP into it, so that the next time it is fused, it’ll award all its stored EXP, plus its own EXP bonus to the hero that it is fused with.
Comparing to the heroes of the same color/rarity, Purple and Gold Eggs can give slightly more EXP than a Rare/Epic hero when used in hero fusion.
Since they are technically not heroes, you cannot deploy them like one. They do not have HP or ATK.