Episodes Suggesting a Conflict
The rivalry between Knife and Marshmallow began out of Knife's boredom after Paper was eliminated due to his piano throwing in A Lemony Lesson. However, Knife still doesn't seem too fond of Marsh, referring to her as "a tiny, weak object".
In The Arena Of Death, Knife said that life was no fun without Paper, and he asked Marshmallow if she wanted to have some fun. She replied "You bet your Marsh I do!" and Knife said that he bet his marsh that since he is very strong and Marshmallow is weak in comparison that he could throw her a very far distance. Marshmallow declined the offer, but Knife picked her up anyway and threw her as she screamed. Knife then questioned how that he could "bet a Marsh" as Marshmallow flew into Balloon and popped him.
In One-Shot Wonder, Knife wanted to cheer Nickel up and comes up with making s'mores, so he grabbed Marshmallow, stuck a twig through her, and roasted her on a fire. Nickel, however, refused to eat her, and said that he could sometimes be a real... something, but the intro cut him off. During the challenge, Knife suggests that Marshmallow being the smallest should go first. Marshmallow, being inactive due to him burning her earlier, doesn't respond, but he helps Marshmallow over the gorge anyway, actually successfully. However, she obviously lost in the second round. (Tug-of-war).
In War De Guacamole, Knife shows Nickel and OJ the wonders of electricity by dragging his feet on the ground and then shocking Marshmallow. He laughed at her, saying that she is so much fun to mess with. As a result, she hit a button on a remote that revealed a launchpad underneath where Knife was previously standing, that made him fly into a gorge, causing him to cut into an apple. She reveals to Nickel that she got the remote from Wal-Mart and that it came with a free cucumber, which Pickle takes.
In Double Digit Desert, when Knife doesn't rejoin, he states that he will be out soon, and when he is, they "can have some more fun". This was likely directed at Marshmallow, referring to the first time they interacted in Episode 3 when he asked Marshmallow if she wanted to "have some fun".
In Inanimate Smackdown, when Marshmallow is eliminated, she is sent to Idiotic Island where Knife threateningly welcomes her, leading to Marsh crying out in anguish and fear.
In Let 'Er R.I.P. , Marshmallow has Bow's Ghost destroy the one thing Knife held dearly; the Dora Doll he got as a prize back in season one's episode 5; The Stacker. Bow's Ghost possessed the doll and Knife angrily asks who is doing this. In the background Marshmallow yells "it's someone you used to constantly bully!" in which Knife responds that this answer doesn't really tell him who it is. So the ghost then says, "Adios!" which is funny because it means goodbye in Spanish and the fact that Dora is Spanish. After destroying the doll Bow's ghost throws him out the window of the mansion, suggesting Marshmallow wanted extra revenge against Knife for him Bullying her in the past.