Liam McAllister is the nemesis of Carl Costello. Liam is the ex-boyfriend of Jem Costello, and was a footballer. His career was ended when Carl broke his leg, potentially on purpose.
Jem and Liam came face to face in October 2010. He and Jem went out together, where she explained why she broke up with him. Liam then began to stalk Jem's brothers, Seth and Riley Costello.
Liam and his brother, Nathan, held a party at their mansion, which was attended by Seth and Riley. Seth sympathised with Liam upon discovering his career-ending injury. Liam took advantage of this and encouraged Seth to get drunk with him and get in the spa pool. He continued to get Seth drunk until Seth passed out. After Seth had passed out, Liam held Seth under the water and left him face-down in the pool. However, Seth survived when Mitzeee Minniver successfully resuscitated him.
After being tricked by Sami, Liam knocked out Seth and kidnapped him and a drugged Riley. Liam and Nathan also kidnapped Carl, tying him up, getting him to stand on a box, putting a rope around his neck, and threatening to kick the box if Riley and Seth, who were locked in the fighting ring, did not fight each other. Sami later grabbed a sword and threatened to decapitate Nathan if they tried to stop her letting Riley and Seth free. After being released, Seth attacked Liam, but Liam and Nathan also attacked Sami.
Liam and Nathan chased Riley, Seth and Carl. They caught up to the trio, who were armed with a loaded gun. Riley threatened to shoot Nathan dead if they continued to chase after them, but the pair let them leave before going after them again. By the time they had caught up to them again, the trio had ditched the gun and were at the top of an abandoned warehouse. The quintet ended up on the roof of the warehouse, on top of unstable scaffolding. When Liam saw that Nathan had been pushed from the scaffolding by Seth (in defence of Riley), and was hanging from a scaffolding pole. Liam tried to lift Nathan back up but the floor began to give way so Nathan let go, falling into an oil tank and dying after being impaled on a spike. The police soon arrived and arrested Liam.
Liam was released and returned to the village. He reunited with Jem, and tried to ruin Carl's life by getting Carl to sleep with Mitzeee. Liam ended up sleeping with Mitzeee, and Jem broke up with him.