The Sun-god and Gilgamesh's main go-to-god.
It must be nice to have a personal god. Shamash, the sun-god, basically seems to hang around to help out Gilgamesh and Enkidu. We first get important signs of this relationship when Gilgamesh and Enkidu are about to set out on their quest: Gilgamesh asks his mother, Ninsun, to put in a good word for him to Shamash, and she does.
Just one of the perks when your mom's a goddess.
Gilgamesh knows which side his bread is buttered on, because he prays to Shamash every night during their journey. Once the friends actually arrive at the Cedar Forest, they are completely terrified—but Shamash steps in to help them, telling them now is the moment to strike. And Shamash actually is a big help in their battle with Humbaba: if he hadn't sent 13 winds to immobilize the monster, it isn't clear that the valiant heroes would have triumphed.
That's why we're a little confused that Enkidu tells Gilgamesh to kill Humbaba quickly, before Shamash knows about it. (This might just reflect Enkidu's ignorance.) Aside from this weird blip, however, the "you-scratch-my-back-I'll-scratch-yours" relationship between Gilgamesh and Enkidu and Shamash continues pretty much normally: when they kill the Bull of Heaven, Gilgamesh and Enkidu present its heart as an offering to Shamash. (Sort of like when your cat leaves a mouse offering on your doorstep. Gee, thanks, Fluffy. We owe you one.)
When Enkidu is dying and spouting curses against the trapper and Shamhat, Shamash speaks out of the heavens, telling Enkidu that he should be blessing Shamhat, not cursing her. If it weren't for her, Shamash says, Enkidu would never have become friends with Gilgamesh—the most important event in his life.
Is Shamash saying this because he likes Shamhat and doesn't like to see her getting cursed unfairly? Or is he mainly thinking of Enkidu, and trying to put him in a better mood before death? Or, is this Shamash revealing to us what we—good Sumerian readers that we are—should recognize: above all else, a good friend is worth dying for? Hard to say. Either way, though, Shamash is one interesting sun-god dude.