“These ancient creatures might be found in the Valley of the Living Rock, but only if they want to be found. Trolls are masters of minerals and yielders of the night sky. They are also overbearing, loud, meddlesome, inappropriate, and heavy―really heavy. But they mean well and love well.”
Most of the trolls appear to be loving and family-oriented, such as when Bulda adopts Kristoff and Sven, and when she and the other trolls try to convince Anna to marry him. Sometimes they seem crude and a bit intrusive, but their affectionate teasing only endears them to Anna. They consider Kristoff and Sven to be part of their family, and they treat them as such.
In the ABC series Once Upon a Time, the Trolls are seen with Pabbie, only in bowl form, in a flashback when Elsa and Anna visit them for to obtain information on the death of their parents and when Anna and Belle visit Pabbie to get answers to their mothers. A statue of a troll is also seen in the present day, when Emma and Hook purchases Elsa.
It should be noted that the crystal necklaces each troll wears represent a gender or position in their culture:
Male trolls wear blue and green crystals.
Female trolls wear pink and red crystals and also wear flowers in their hair.
Pabbie, Bulda's father and Kristoff's adoptive maternal grandfather, is the only troll to wear yellow crystals, possibly signifying his role as the leader.
When they speak, their crystal necklaces light up, as seen in "Fixer Upper".
According to the book The Art of Frozen, the trolls' magical crystals channel the aurora borealis.
In most Nordic countries, trolls were evildoers who eat humans, especially children, or tried to trick them. Across Europe, there were also stories of Changelings and Fetches - when Trolls steal a human child and replace them, sometimes with fairies, and sometimes a log of enchanted wood that becomes 'sick' and dies. But there are also stories where they are nothing more than magical neighbors who borrowed stuff from the farmers, and if the farmers treated them well, they were rewarded with gold or luck. They also loved children, so the easiest way to make friends with a troll was to be nice to their kids, and it was said that they took care of the human children that were abandoned in the woods. The stories include that the Trolls were particularly attracted to blond hair and beauty, and often treated the child as if it were truly one of their own. In return, they gave the child gifts such as strength, stamina, or an affinity for the wild.
Contrary to the negative portrayal of trolls mentioned above, these trolls are portrayed as a benevolent force and are very helpful to those who need it.
Interestingly, their adoration for children is actually briefly hinted at by Bulda in the Broadway musical when she tells Queen Iduna to "Call on us anytime, we love children and even raise a few strays ourselves."
It is possible that these trolls are based off the robbers that appear in the original tale of The Snow Queen. In the original story, these robbers capture Gerda (Anna), who becomes the new playmate of the youngest robber, the Robber Girl (whom Kristoff is based off of), who later releases her after hearing Gerda describe her mission to rescue Kai from the Snow Queen (both of which Elsa appears to be based on).
In The Snow Queen, off of which Frozen is based, the devil, took the form of a troll who was headmaster at a troll school. He and his pupils attempted to take a magic mirror that magnifies people's bad and ugly aspects into heaven to trick the angels, however they dropped it, it fell back to Earth shattering into billions of pieces. The splinters are blown by the wind all over the Earth and get into people's hearts and eyes, freezing their hearts like blocks of ice and making their eyes like the troll-mirror itself, seeing only the bad and ugly in people and things.
The trolls did not appear in the Broadway adaptation. They were instead replaced by a group of creatures (also from Scandinavian lore) known as the "hidden folk".
Ironically the portrayal of the "hidden folk" in the Broadway musical depict how Trolls in traditional Scandinavian folklore (most notably Norwegian Trolls) were depicted. Trolls in Scandinavian folklore were also referred to as being part of the Huldrefolk (literally meaning "Hidden Folk").
As a reference to them being made of rock, the three main trolls (Bulda, Cliff, and Pabbie) have names that are puns of or allude to rock-related terms. Bulda's name sounds very much like "boulder", her husband Cliff's name refers to the term of the same name, which is a steep rock face, and Pabbie's name is an allusion to "pebble".
↑Lee, Jennifer (September 23, 2013). "Frozen Final Shooting Draft", pg. 83