Given this text about Heracles' twelve labors, construct a list of characters that either help him in his quest, how, and why.

Context: 1. Slay the Nemean Lion
Heracles defeated a lion that was attacking the city of Nemea with his bare hands. After he succeeded he wore the skin as a cloak to demonstrate his power over the opponent he had defeated.
2. Slay the nine-headed Lernaean Hydra
A fire-breathing monster with multiple serpent heads. When one head was cut off, two would grow in its place. It lived in a swamp near Lerna. Hera had sent it in hope it would destroy Heracles' home city because she thought it was invincible. With help from his nephew Iolaus, he defeated the monster and dipped his arrows in its poisoned blood, thus envenomizing them.
3. Capture the Golden Hind of Artemis
Not to kill, but to catch, this hind that was sacred to Artemis. A different, but still difficult, task for a hero. It cost time but, having chased it for a year, Heracles wore out the Hind. Artemis intervened, but as soon as Heracles explained the situation to her, she allowed him to take it, and he presented it alive to Eurystheus.
4. Capture the Erymanthian Boar
A fearsome marauding boar on the loose. Eurystheus set Heracles the Labour of catching it, and bringing it to Mycenae. Again, a time-consuming task, but the tireless hero found the beast, captured it, and brought it to its final spot. Patience is the heroic quality in the third and fourth Labours.
5. Clean the Augean stables in a single day
The Augean stables were the home of 3,000 cattle with poisoned faeces which Augeas had been given by his father Helios. Heracles was given the near impossible task of cleaning the stables of the diseased faeces. He accomplished it by digging ditches on both sides of the stables, moving them into the ditches, and then diverting the rivers Alpheios and Pineios to wash the ditches clean.
6. Slay the Stymphalian Birds
These aggressive man-eating birds were terrorizing a forest near Lake Stymphalia in northern Arcadia. Heracles scared them with a rattle given to him by Athena, to frighten them into flight away from the forest, allowing him to shoot many of them with his bow and arrow and bring back this proof of his success to Eurystheus.
7. Capture the Cretan Bull
The harmful bull, father of the Minotaur, was laying waste to the lands round Knossos on Crete. It embodied the rage of Poseidon at having his gift to Minos diverted from the intention to sacrifice it to himself. Heracles captured it, and carried it on his shoulders to Eurystheus in Tiryns. Eurystheus released it, when it wandered to Marathon which it then terrorized, until killed by Theseus.
8. Steal the Mares of Diomedes
Stealing the horses from Diomedes' stables that had been trained by their owner to feed on human flesh was his next challenge. Heracles' task was to capture them and hand them over to Eurystheus. He accomplished this task by feeding King Diomedes to the animals before binding their mouths shut.
9. Obtain the girdle of Hippolyta, Queen of the Amazons
Hippolyta was an Amazon queen and she had a girdle given to her by her father Ares. Heracles had to retrieve the girdle and return it to Eurystheus. He and his band of companions received a rough welcome because, ordered by Hera, the Amazons were supposed to attack them; however, against all odds, Heracles completed the task and secured the girdle for Eurystheus.
10. Obtain the cattle of the monster Geryon
The next challenge was to capture the herd guarded by a two-headed dog called Orthrus, which belonged to Geryon; a giant with three heads and six arms who lived in Erytheia. While travelling to Erytheia, he passed through the Libyan desert and was so annoyed by the heat he shot an arrow at Helios, the sun. Helios, impressed, lent him his giant cup which Heracles used to find Orthrus, the herdsman Erytion and the owner, Geryon. He killed the first two with his club and the third with a poisoned arrow. Heracles then herded the cattle and, with difficulty, took them to Eurystheus.
11. Steal the golden apples of the Hesperides
Hercules stealing the golden apples from the Garden of the Hesperides
These sacred fruits were protected by Hera who had set Ladon, a fearsome hundred-headed dragon as the guardian. Heracles had to first find where the garden was; he asked Nereus for help. He came across Prometheus on his journey. Heracles shot the eagle eating at his liver, and in return he helped Heracles with knowledge that his brother would know where the garden was. His brother Atlas offered him help with the apples if he would hold up the heavens while he was gone. Atlas tricked him and did not return. Heracles returned the trickery and managed to get Atlas taking the burden of the heavens once again, and returned the apples to Mycenae.
12. Capture and bring back Cerberus
His last labour and undoubtedly the riskiest. Eurystheus was so frustrated that Heracles was completing all the tasks that he had given him that he imposed one he believed to be impossible: Heracles had to go down into the underworld of Hades and capture the ferocious three-headed dog Cerberus who guarded the gates. He used the souls to help convince Hades to hand over the dog. He agreed to give him the dog if he used no weapons to obtain him. Heracles succeeded and took the creature back to Mycenae, causing Eurystheus to be fearful of the power and strength of this hero.

Answer: - Iolaus, who helps him kill the Lernaean Hydra, since he is his nephew
- Artemis, who allows him to take the Golden Hind, after Heracles explains his situation
- Prometheus, who tells him that his brother Atlas knows the location of the garden of the Hesperides, in return for being saved by Heracles