Keltset comes from Norbane, one of the larger Troll cities in the far northern Charnal Mountains. His family, the Mallicos family, was a very old and honored line in the Northland. However, his family were all killed in an attack that even distant Troll villages ended up hearing about, which was testament to its grisly nature.
The attack was supposedly committed by Dwarves who had attempted to loot the Mallicos home. However, it was really minions of the Warlock Lord who killed the family: Skull Bearers came to Norbane almost a year before Keltset met Panamon and Shea, and they seized control of the government and ordered the Troll armies to accept their command. The Skull Bearers managed to convince most of the city that Brona had come back from the dead, and that he had survived for millenia and could not be killed by mortal means.
But the Mallicos family was one of the ruling families in the city who refused to submit to the Skull Bearers, and they demanded that the city stand firm against the Warlock Lord. Keltset's word in particular carried a lot of weight because he was a Troll of especially high standing: he was a recipient of the Black Irix, the greatest recognition for honor that a Troll could receive.
The Warlock Lord had the entire Mallicos family killed except Keltset, whom he brought to his fortress in the Knife Edge. The story of Dwarf looters was spread as a deception to inflame the Trolls into joining the Warlock Lord's invasion of the southern lands. Keltset's voice was burned out to prevent him from telling his story to others, but he managed to escape before they got him to the prisons and he learned sign language to communicate.
Keltset wandered southward from the Skull Kingdom until Panamon found him and they became friends, and the Troll waited for his chance to return to the Northland. Although Panamon knew sign language, Keltset did not reveal his history to him.
Keltset gets the chance to return when he and Panamon meet Shea Ohmsford, and he decides to help the young Valeman in his quest. The Troll's past becomes a matter of curiosity to the two other men when they encounter a Skull Bearer who recognizes Keltset and expresses fear. Pressing on, the trio are captured by a party of Trolls, and Keltset is judged for bringing Southlanders into the Troll territory. Keltset stands down when confronted by the party of Trolls and allows himself and his companions to be taken by the party, worrying Panamon and Shea as to where his real allegiances lie.
However, once placed in front of the entire Troll village for questioning, Keltset reveals the Black Irix to his captors, and the symbol proves to the village that he is honorable to the point of being incapable of treachery. Having proved his honor, Keltset reveals his identity and his past to everyone. He says that he returned to the Northland because he learned of Brona's fear of the power of the Sword of Shannara and the legend that a son of the House of Shannara would appear to take up the Sword. Keltset successfully urges the Trolls to help him, Panamon, and Shea travel to the Skull Kingdom, promising that once inside the fortress Shea will destroy the Warlock Lord.
Once inside Skull Mountain Keltset is unable to defend Shea against Brona, but he survives the former Druid's wrath and flees Skull Mountain with his friends. Before they can escape, the mountain begins to collapse. Keltset holds the roof of their tunnel in place long enough for Panamon and Shea to escape, but is crushed beneath the crumbling mountain, forcing his friends to leave him behind.
Keltset is described as being massively built, with rough, thick, bark-like skin the color of meat "well-done, but not yet charred." Like all Trolls he has only three fingers and a thumb on each hand.