Humbaba is the monster who guards the Cedar Forest.
Being a monster and all, he isn't the most complicated character, although he can do a neat trick where he changes his faces, and apparently it is pretty hideous. He is described as having seven layers of mystical armor, though he only wears the full seven layers at certain times of the day.
Why's that? We don't know. But lucky for everyone (except him), he's only wearing one when Gilgamesh and Enkidu attack. Even so, the friends only triumph because the sun god Shamash sends down 13 winds, which strike Humbaba from all directions and immobilize him.
When Gilgamesh finally has Humbaba beat, Humbaba begs for his life. Is this a pathetic move on the part of a monster? Not really. See, Humbaba isn't technically a "bad guy." He doesn't just hang in the Cedar Forest threatening everybody because he's some kind of selfish ogre who wants the trees all to himself. Basically, his deal is that he was appointed by Enlil, the king of the gods, to be super scary and guard the Cedar Forest. That's right: he's just doing his job guarding the trees from humans who would like to cut them down.
(We at Shmoop would like to remind you that this story takes place in modern-day Iraq—not exactly the place you think of when you imagine huge forests of trees. So, perhaps Enlil was right to put Humbaba there. Humbaba is dead, and Iraq looks like this. Although, to be fair, they do have to walk a really long way before getting to the forest.)
Anyway, Humbaba appeals to Gilgamesh's sense of mercy when he realizes there is no getting out of this one. Then he lays it on thick, saying he he'll become Gilgamesh's servant and cut down whatever trees Gilgamesh wants him to. (Pretty quick switch of loyalties, Humbaba.)
Then he appeals to Enkidu, reminding Enkidu about how he was only being a big bad monster because that's what Enlil appointed him to do. But Enkidu isn't moved, and keeps egging Gilgamesh on to kill the monster. Once Humbaba realizes that he's toast, he curses Enkidu. And who can blame him, really? In fact, he prays that Enkidu will die before Gilgamesh. (Does this explain Enkidu's quick demise?)
What does all this tell us about his character? Well, he's a monster. It's not his fault, and he has no real sense of loyalty. We're not too into merciless killing, but it's hard to get worked up about his death.