"Rahhh!" *In Chapter 4: "Safe Haven", Yarnaby was found to be very aggressive and hostile. When the Player gets inside of a cell, Harley Sawyer calls Yarnaby and opens the cell door, causing Yarnaby to leap out. He crashes into the cell's window, pauses before attacking again, hitting the window with his head several times, scratching it, screaming, and shouting. As shown in his cut-out quotes, Yarnaby is supposed to have a playful personality. He tends to rhyme his name with other words and enjoys being brushed. He is also described as having a heart of gold, showing that he has a pure and kind nature and his jingle also shows that he has a protective nature.

According to his weekly check-up, as well as the Prisoner Profile, Yarnaby was originally stated to be docile, agile, and highly impressionable. Unlike the other experiments, the Bigger Bodies version of Yarnaby showed no signs of hostility toward the scientists and staff. Furthermore, he displayed a peculiar semblance of obedience and loyalty towards certain security officers and scientists, including Dr. Harley Sawyer himself (similar to how CatNap was loyal to the Prototype). During his isolation sessions with The Doctor, Yarnaby developed an affinity with the latter, perceiving him as his master, which was exactly as Harley intended, though he never cared for Yarnaby.

Following Harley Sawyer's removal, Yarnaby lapsed into an unresponsive state fueled by grief and his wishes for his master's return.*