Sleuther was a very aggressive dragon in the dragon fighting arena. He was known to kill other dragons, as Ryker stated "if the Triple Stryke kills that Night Fury." In a fight with Toothless, Sleuther was spared by him as he had won the fight and learned from the Night Fury that dragons don't need to kill other dragons. Sleuther gained a lot of trust in Toothless, as he wanted to live on the Edge, but he showed some uncertainty of leaving the Dragon Fighting Arena as he lived in captivity for a long time. Still, when Toothless gave him encouragement to be free, the Triple Stryke took off with the Dragon Riders. Later, Sleuther begins enjoying the freedom by spinning and flying openly. He then sticks out his tongue like Toothless, showing that he is a playful dragon, not unlike the Night Fury.
Though he was released from the Dragon Arena thanks to the riders, he still showed hostility towards them. This is most likely due to his species' aggressive nature. Even though Sleuther and Toothless had formed a close bond, he didn't hesitate in attacking him. This is likely the result of his time in captivity, where he was forced to partake in many battles to the death, causing him to still be wary of humans and dragons, even those he was close to. It's also possible that Sleuther was defending his territory, and that the riders didn't realize the Triple Stryke had a territory on the Edge. This indicates that the species is very territorial.
However, after Astrid discovered Sleuther responded to clicking sounds, he appears to have calmed down, which made him more susceptible to training. He was quick to assist Dagur after the Berserker saved him, which could mean that Sleuther has made some amends with his past, as he was returning the favor to Dagur.
Sleuther has been shown to understand human language, as he threatened Snotlout when the boy mocked his rider. He also understands Dagur's commands and is very obedient. He seemed to know that Dagur and Mala were in love with each other, being a bit disgusted by the situation and trying to shed his eyes and ears from the pair's sapy actions.