The crux of McAllen's plan is to place a corrupt Guardian in the Tower and use him to give McAllen control over both Stark and Arcadia. To this end he captured Gordon Halloway - a guardian-to-be - and subjected him to the Vanguard experimentation that split him in two. Gordon eventually became the head of McAllen's secret police, and effective second-in-command: McAllen even trusted him to capture Cortez on his own (with a "small army"). McAllen also uses his technology to facilitate the creation of Warrior Shifters, to hold Cortez prisoner and remove the Red Jewel that he holds.
McAllen is a thoughtful, scornful character. He is a calculating man "but not cruel", as he tells April. He appears to have a marked dislike of April for some reason - possibly because, although she is also a member of the Draic Kin, she was raised in a human family. He says he has spent a considerable amount of time and money tracking April, "before you were born, before I even knew your name." This is interesting: it implies that McAllen was searching for the one foretold in the prophecies, but didn't realise she was going to be one of the Draic Kin at first. This may be the source of his antagonism.
Despite the fact they appear to be roughly the same age, McAllen has a marked hatred of his brother, and mentions to April at one point that Cortez "broke the rules. He brought you into -- he broke the rules." However, he does try to convince Cortez to join him, saying "the time of dragons has come", but Cortez scorns him and the two do battle. McAllen is last seen in his true form as the Green of the Kin, falling from the roof of the MTI Building. His true fate is unknown.