As a young man, Caellach became a mercenary in order to get away from his hometown in Jehanna. In this mercenary troupe, Caellach eventually came to meet Joshua, who had also abandoned his country in order to travel the world as a mercenary. Eventually, Caellach's drive to succeed bolstered him upward to become one of Grado's generals.
Through the course of the game, Caellach is responsible for the destruction of two Sacred Stones. Alongside General Selena, Caellach infiltrates Frelia and destroys their Sacred Stone first. Afterwards, he is charged with eliminating the Stone in his home country, Jehanna. As events unfold, Caellach is seen using less than savory ways of accomplishing his missions, including holding Queen Ismaire hostage in order to get Carlyle to rebel to get his way. Despite his promise to Carlyle that he would not hurt her, Caellach murders Queen Ismaire of Jehanna, in order to find its Sacred Stone.
Ch. 15, Caellach, along with Valter stage an attack on Eirika in the Jehanna dunes. The player may choose to fight Caellach with Joshua. In Ephraim's story, Caellach reveals that he and Joshua once worked together, and tries unsuccessfully to persuade Joshua to come over to his side. In Eirika's story, Caellach reveals his ambitions and his knowldege that Joshua is a prince. Joshua, in turn, remarks on Caellach's murder of his mother. Caellach tries to persuade him not to hold a grudge against him, after which Joshua angers him by responding that he should not hold anything against Joshua for killing him. By the end of the chapter, Caellach was dead.