Leir is the younger brother of Nuada and the Dagda. He is a highly accomplished warrior among the Celtic gods and has often led them into battle. However, his own brash and impetuous nature have prevented him from ever assuming the position of leader. He will often choose to fight when it is either unnecessary or better to retreat. He has a fierce rivalry with his fellow god Bodb Derg, the god of war, and something of a friendly rivalry with the Asgardian thunder god Thor. He has fathered several gods and eventually turned his duties of the god of the sea over to one of them. He had a long-standing hatred of the Asgardian gods due to conflicts between their mortal worshipers.
During modern times the Leir was attacked in Avalon, the home of the Celtic gods, by a beast foreign to the realm. Leir mistakenly thought the attacking beast had been sent by the Asgardians when he next saw and then engaged Thor in battle after he arrived in the Celtic gods' realm while in pursuit of that beast. The two set aside their differences upon realizing the beast was from neither of their realms and had instead been sent by the Egyptian death-god Seth. Leir later convinced the Dagda to end hostilities with Asgard. A short time later Dagda dispatched Leir and his fellow Celtic warriors to aid Asgard in their war with Seth. Seth was a threat to all life and would eventually come for the Celtic gods, so it made sense for the Celtic gods to put aside their differences with the Asgardian gods and unite in a common cause. Leir also believed he owed Thor a debt and wished vengeance for Seth's earlier attacks.
Leir later attacked a camp of Formorians, another race from the realm of Celtic gods, who are ancient enemies of the Tuatha De Danaan. The battle was broken up by the Dagda before a war could be started. Leir had fallen in love with the Asgardian Sif, so he tricked the Dagda into sending him to Asgard to pursue her. He aided her in finding Thor after being told he had to best Sif's champion in battle. He thought it was Thor, but it turned out to be Sif herself. Underestimating her fighting skills and unwillingness to battle a woman as he would a man, Leir was humiliated by Sif in a battle witnessed by his fellow Celtic god Caber. After this humiliating defeat, Leir and Caber returned to Avalon.
