Humbaba was the child of Utu, birthed with the purpose of being a "complete human being." With the exception of Ishtar and Ereshkigal, all the gods believed that Humbaba was such, Enkidu later commenting that such a mindset showed that it was too late to stop their parting from the humans of the world. Humbaba was actually an inhuman beast of the gods that lived as the guardian of the Cedar Forest. Upon their manifestation into the world, Enkidu was a formless clump of mud, so they were told to seek out the "complete human" to copy its form to be able to interact with humans. Enkidu did as instructed, becoming its friend while they still lived in the wild. Enkidu made it a garden by bending the laws of the earth, and it wore a circlet of flowers on its head taken from the garden.
Enkidu was later sent to find Shamhat, as their humanity was said to still be lacking, and they parted from Humbaba. After Enkidu fought and befriended Gilgamesh, they both traveled to the forest to slay it. Although he cared not for the people currently suffering under his reign at the time, Gilgamesh considered Humbaba an evil of the earth that was necessary to defeat to keep the people from starvation. Enkidu wondered why it wore the circlet, and Gilgamesh replied, "For its innocent heart was it was all the more joyous for a simple wreath." As Gilgamesh returned to Uruk, Enkidu left a second circlet behind for their friend.
Humbaba is described as a "thing" that can be called neither "he" or "she." Enkidu sometimes addresses it as "she" due to its personality being like that of a young girl. It wore a circlet of small, pale flowers on its head. The "Fate/Zero Animation Visual Guide I" has a portrayal of the battle between Humbaba, Gilgamesh, and Enkidu in the "Legends of Heroes" section. As with the other images in that section, it may not reflect the character's actual design.
Although a monster, Humbaba possessed the innocent heart of a young girl. It found great joy in the circlet of flowers from the garden made by Enkidu. Upon first meeting Enkidu, it lets out "a cry of perpetual hate and unending resentment for the whole world."
Nasu makes an aside on his blog pondering if the runaway Ibaraki-douji may have actually been holding back the resurrected sacred beast Death Huwawa (デスフワワ, Desu Fuwawa?) from joining the war in the Babylon Order.
Humbaba is said to have possessed claws sharper than anything on earth, surpassing even Enkidu. It required the combined might of Gilgamesh and Enkidu to defeat it.