Gruff used to be an ordinary wild dragon, until he was caught in a tree snare by one of Drago's Dragon Trappers, and the incident blinded him. Fortunately, Valka and Cloudjumper had found him. Discovering who had maimed him, Valka asked the Bewilderbeast for aid and the king of dragons attack Eret's fortress and destroy it. Despite being blind, he still lives a happy life in Valka's Dragon Sanctuary.
Gruff appears to be very close to Valka as he seems to enjoy her presence as he was saved by Valka to be very thankful, turning yellow when she strokes him.
Gruff is apparently very sensitive and shy. This was largely due to past incidents and traumatic damages he carries. However, this sensitiveness helps others to know subtle movements in nest mates' emotions, and with his abilities native to his species, the Hobblegrunt can be very useful and helpful as part of the warning system for the Sanctuary. According to Dragons: Rise of Berk, Gruff is very intelligent and can be trained to find its way to Berk, showing that he can fit in rather well with other dragons. Gruff is also mentally strong to live happily without sight.
Gruff is so far the only known member of the Hobblegrunts. Due to the body size, he seems not to be a matured individual. He has a big frill at the back of his neck, clawed wings, sturdy nadder like legs, and short arms. His eyes are dull-colored caused by the snare that blinded him. He has long teeth protruding from the lower mandible. He resembles a theropod dinosaur like a Speed Stinger does. He changes color based on its mood, like all Hobblegrunts, the only parts that are always the same coloration are the blue spots he has all over his body.
It might be that Valka, on her own, named all of her rescued dragon species or she may only give names to the ones that were her closer friends and allies such as Gruff.
In Dragonpedia, it is shown that Gruff now has the ability to show his mood and to change the mood of dragons around him by changing his color and vibrating his crown.
Gruff's card in Dragons Challenge Card Game states that he is a Horned Hobblegrunt. What this means remains to be seen.