Lambert was a witcher from the School of the Wolf, having been trained by Vesemir. While he was trained at the same time as Geralt and Eskel, he had a rather cynical outlook and didn't take kindly to outsiders, that is, non-witchers.
Lambert arrived at Kaer Morhen as a child, at the same time as Geralt, Eskel, and Remus. The group was trained under Vesemir and underwent the Trial of the Grasses successfully, but soon after Kaer Morhen was attacked by an angry mob of both people and monsters in 1165. However, thanks to Illyana Zerbst and Geralt's snooping, the children managed to escape outside and away from the keep. Vesemir, the sole witcher to survive the massacre, soon found the tired out children and took them under his wing to care and train.
Upon Geralt's return, accompanied by Ciri, Lambert told them the tale of the time a farmer hired him to kill mora until he learned from the farmer's wife that she was simply having an affair with the field hand. Later that evening, Lambert barged into Ciri's room with the whores they invited to the den and locked them inside for safety after realizing that they were under attack.
Tension had grown between Lambert and Geralt, who had killed Eskel after he got infected by a leshy, but Geralt reminded him that what he killed was no longer their brother and that bitterness wouldn't help find the leshy that did actually kill him.
With Coen at his side, Lambert decided to take his frustration out on Ciri, antagonizing and taunting her over the fact that she was practice fighting against a straw dummy, whereas real witchers trained on the Pendulum, which was an extremely dangerous obstacle course that knocked Ciri on her back multiple times, though she refused to give up and kept trying to complete the course, even as Lambert advised her to go home. Due to her determination, Lambert start to encourage and advise Ciri on how to finish the course. On her final try, she nearly cleared the course but was unable to stick the landing at the end.
Lambert and Coen greeted Triss as she arrived at Kaer Morhen to train Ciri. The following day, the duo heckled Ciri, who entered the dining hall wearing a bright-colored shirt with a flower in her hair to match Triss. Upset, Ciri, stormed off, which warranted a scolding from Triss.
After Vesemir and Triss were attached in the lab, Lambert began to wonder what a mage needed with a witcher, but Triss insisted that he was not one of her kind. Fire magic was off-limits for them. Whoever he was, he wasn't working for the Brotherhood.
Hunting Down Ciri
Lambert and Coen received word from Vesemir that Ciri was possessed by Voleth Meir and while under her control, she murdered several of their brothers whilst they slept. Fueled by their rage, they hunted Ciri down and found her in the dining hall, where she unleashed a scream so powerful that it cracked the medallion tree, revealing a monolith hidden inside. She emitted yet another scream, shattering the monolith and opening a portal to release two basilisks. While the witchers eventually managed to kill the beasts, they lost two more brothers in the process. Yennefer then slit her wrists and summoned Voleth Meir into her body, allowing for Ciri to regain control and portal the Deathless Mother back to her home sphere.