"Mischief" (aka "Pink Guy" or "Smile") as referred to by fans is a recurring character in the Interface series and seems to be a key figure in the overlying plot. They have been seen in many episodes and is often accompanied by "Blue Guy".
"Mischief" is believed to originally have been an American Human within the US Navy, although their form was altered by their involvement in the 1943 Philadelphia Experiment. The Experiment has been subject to much speculation and conspiracy. Within the Interface universe, the ship was teleported from Philadelphia to New York. The process also caused rather negative effects, like the crew fusing to the hull. Mischief's previous being was destroyed as a result of the incident and they instead became the creature they appear as now.
Although "Mischief" appears mainly as a pink, serpentine creature they are known to take many other forms such as a bus or a "beautiful sunflower". It is believed that this shapeshifting power may have been gained through Cerebral Electricity, a mysterious energy which appears throughout the series. It is unclear the full extent of power gained by Mischief through the use of Cerebral Energy as well as how many possible forms it might take. In Episode 8 "Mischief" appears to "Blue Guy" by flying through an open window as it claims to be a "Bird" but also states that "I haven't quite figured it out yet." This may suggest that "Mischief" requires practice to properly take the form of an object and replicate its behaviour. Mischief almost always takes the form of pink objects, with the notable exception of the "beautiful sunflower" in Episode 5, though he explains to Henryk in Episode 11 that "The reality is, I don't want to [change colors]. I choose to be this way."
Not much is know about "Mischief's" personality, but if its name suggests it may be of a mischievous, humorous, yet compassionate kind. In Episode 8 Mischief appears before Henryk as a bird and jokingly tweeting and telling him that it's "trying to be a bird". While later questioning him that "I bet the nurses in the next room over are wondering how a man over a hundred years old has skin smoother than theirs." Before taking the form of a health and beauty magazine and telling Henryk saying quote "I did that as a joke, but you're pulling it off." In Episode 9 Mischief takes the form of Henryk's coat and hat in a Japanese Sushi shop later joking to the chef octopus "Isn't that a bit cannibalistic of you to be chopping up sea creatures?" Mischief can also be quite of an interventionist in the sense that he confronts Henryk by questioning him on what did he lose to gain what it seems to be either eternal life or a vastly long lifespan than a normal human has before Henryk collapses from a flashback to his life with his family. In the Adult Swim segment "Stranger Danger" Mischief appears before a boy inside a drain telling the boy that he should cover a cut on his face with medical ointment then giving back a paper boat. After which he asks the boy did he receive a vaccine shot telling that it's important to in order to prevent the spread of deadly diseases. This may tell that Mischief is compassionate being by warning other the dangers of diseases.