Isaac Lahey was a former beta of Scott McCalls alpha pack, before he left the pack to morn the loss of Allison Argent after her murder.
Once the son of an abusive father, he was turned by Derek Hale after feeling powerless in his home life. Eventually, after the death of his father, he and Scott McCall grew to be close friend, and subsequently became very close with Scott's ex girlfriend, Allsion. The three of them remained very close until Allison's death, when Isaac left with Allison's father Chris, for France.
After an adventure in the Hunter Capital, Céline and gainign new hunter abilties, Isaac reunites with an old class mate, A former pack mate, and an old enemy. Eventually, the four form a dangerous team and resolve to return to Beacon Hills to stop the Nemeton and the various Royal Breeds who are converging on it.