Boulder is a supporting antagonist and the main antagonist of the Rusty and the Boulder YouTube Poops.
Boulder is very talkative as he always speaks when he gets the chance. He is also very territorial as he aims to protect his mountain from a quarry being built there and will do anything to keep his home. Boulder ended up becoming more malicious in what he did as he shows no remorse towards killing anyone even if they opposed the quarry's building and he insults engines who run away from him. Boulder also holds long grudges as after The Fat Controller sent him to Armenia and he returns to Sodor, his main plot is to kill him for stealing his home and sending him away. He confuses the engines as their memories were wiped when he mentions the past but is impatient with them when they don't remember. Boulder is also quite skilled as he managed to go from chasing Rusty backwards to going behind him on a separate track, proving he can control himself and it's not just physics that's moving him.
Despite being evil, Boulder could also be considered sympathetic as he was defending his homeland from invading forces who wanted to turn it into a quarry for profit without consulting him. Boulder also suffered The Fat Controller's wrath the worst by far as his second attempt at taking his home back resulted in him being biofused into a truck.
Boulder makes his debut as the main antagonist. He has stood alone on a mountain for several years and is quite proud of it. That all changes when workmen arrive to build a quarry there by orders of The Fat Controller. Boulder spouts a face and demands they keep away from the quarry but to no avail. While the workmen don't seem worried, Rusty was and calls the quarry dangerous because of him and accuses him of watching him. Percy however remains sceptical claiming boulders don't have eyes. No one checks on Boulder during the work at the quarry and there's an indication he will fall off it.
When it rains, workmen leave work and Boulder is alone again (albeit with Thumper left there). Rusty however goes to quarry and is nervous about Boulder. A rock falls onto the rails as a warning sign from Boulder to go away and so Rusty obliges. Boulder still remains annoyed however as people will still come to his mountain once the rain stops.
More machinery comes to the quarry the next day and Boulder decides he had enough and fell onto Rusty although it was later changed to him landing on the rails and proceeding to chase Rusty. Boulder begins getting talkative and threatens Rusty throughout the chase. He starts chasing him backwards but ends up behind them on a siding Rusty fled to in hopes of Boulder passing by. Rusty then finds another siding but Boulder goes down the line. Frustrated at his failure, he threatens to come back for Rusty later.
Boulder then chases Skarloey as he believes one engine is as good as another but Skarloey escapes and Boulder destroys a water tower. A confused Rheneas is also chased by Boulder. Rheneas jumps off a bridge but Boulder follows him. Rheneas is later shown to have escaped.
Boulder is then at the yards and Percy sees that Rusty was right. Boulder then said he will have fun killing Percy but he misses and causes an explosion at the sheds. The Fat Controller inspects the damage and closes the railway. He was going to reopen it but he was going to keep the quarry and tells Boulder that he should have the land. As such, he deports Boulder to Armenia, a mountainous country, where he won't cause trouble on Sodor anymore.
It's revealed that The Fat Controller had wiped everyone's memories of the events and only he, Jackninja5 and Thumper remember the events. However, due to a revolution in April 2018, the new government of Armenia didn't want Boulder to be endangering the lives of civilians so Prime Minister Nikol Pashinyan had him deported back to Sodor. Boulder is proud to be back and sings AC/DC's Back in Black to emphasise it.
Thumper, remembering the events, is reluctant to work despite pressure from the workmen. He tries to get the workmen to remember but they think of him being stupid for thinking so. Rusty is nervous about Boulder but doesn't remember him but is threatened by a rock thrown on the tracks and sees Boulder reveal his face.
Next morning, he strikes again. Boulder rolls down but instead of landing on Rusty, he lands on the rails, asking him where The Fat Controller was and recollecting the times he and Rusty had. Rusty doesn't remember but Boulder believes he is simply denying it and tells him to remember. When Rusty can't, Boulder proceeds to kill him but he runs away again. Boulder then chases Rusty into the same siding but instead of vowing to get Rusty, he focuses entirely on killing The Fat Controller.
Skarloey is chased into a siding as well and Boulder demands he tell him or Skarloey gets killed. Skarloey doesn't know so he leaves but not without knocking over a water tower. He chases Rheneas again and mocks him as a coward for running away. Boulder then proceeds to the same sheds he destroyed and again notes he will have fun killing Percy. He misses again and causes another explosion. Eventually, The Fat Controller finds out and says that he and Boulder should meet again alone.
It is later revealed that The Fat Controller had biofused Boulder into a truck, much to his horror. It is currently unknown where Boulder is among the trucks as he looks like a regular van now.