His name "My Arm" derived from when he was just a baby. Trolls are named after the first thing they - successfully or not - ate. Since he first tried to eat his dad's arm, his dad decided to name him "My Arm".
My Arm is introduced to the player at the very beginning of the quest. Burntmeat learnt that goutweed could be grown again as it was previously thought to have been botanically extinct after the Trolls had picked it all for use as a cooking ingredient. Burntmeat discovered that it was possible to regrow the weed when he was interrogating a captured adventurer who he was preparing to boil into a stew, after he learnt about this he cooked him anyway. Burntmeat encourages the player to speak with My Arm to begin to process of regrowing the herb.
My Arm is an avid farmer but unfortunately he has little knowledge of how farming actually works, he has planted several objects on the roof of the Troll Stronghold, such as crates, rocks and a goat, which did not grow, much to his confusion. He asks the player to grab a goutweed from the troll cooks in Death Plateau, and after the player obtains it, eats it, much to their disappointment. He later accompanies the player to Tai Bwo Wannai village to get more goutweed.
However My Arm is a little too enthusiastic and causes much havoc when the player attempts to cultivate the herb.
Upon getting the goutweed, My Arm recommends for the player to bring combat gear from the bank before returning to the Troll Stronghold. My Arm later plants the gout tuber, and with help from the player, successfully grows some goutweed. However, a Baby Roc appears and the player kills it. My Arm tells them that that was just the small one, and the Giant Roc later appears. My Arm assists the player in killing it, and after it dies, picks some goutweed and gains a Farming level.
After completion of My Arm's Big Adventure, My Arm will watch over a herb patch that a player can use for free. My Arm will ensure that the patch will not become diseased.
My Arm plays a role in the King of the Dwarves quest, where he is found in the Barendir tunnels. When they enter, Pretty Flower thinks they are a tall dwarf, but My Arm tells him that they are a human instead, which Pretty Flower does not eat due to believing that they may be poisoned. My Arm tells the player that he came to help Pretty Flower grow new foods, since the troll warlord is afraid of someone poisoning his food. My Arm later gives the player a Pretty flower to give to the troll warlord.
My Arm later appears on Yu'biusk along with Burntmeat during the The Mighty Fall quest. He says that he came to the Kyzaj Tournament in support of the player instead of the troll champion Lol, as he would rather cheer for a friend. My Arm was initially unaware of Bandos' death, and only finds out when Burntmeat mentions it.
When using goutweed on My Arm, he will say "Stop it - you make My Arm hungry".
In the beginning of the quest My Arm's Big Adventure, when joking about My Arm's name, he tells you about how he sees players with numbers and weird names. This is claimed a joke by Jagex considering how many players have numbers in their usernames.
My Arm tried farming before the quest; he tried "growing" a trolley, a crate, rocks, a tree branch, some fish, and a goat.
Once you return from Ardougne, My Arm says "Dolce domus" and claims that "It means home, sweet home in some old language". This language is actually Latin, which he knows because he is an "ed-yu-cat-ed troll".