Excellus serves Walhart as his tactician for battles, but was secretly manipulating him. Excellus' skills lie not in war tactics, but his ability to con and deceive others into joining Walhart's army through blackmailing and threats. Excellus cares only for himself, Grima and seizing power, even if it means abandoning his superiors. However, despite his cowardly nature. Excellus was secretly a devout member of Grimleal, and was manipulating Walhart to fail in his quest to destroy the Grimleal. Excellus also took a deal from Aversa which offered him the throne of Plegia if he was able to take the Fire Emblem from Chrom.
At Fort Steiger, Excellus managed to threaten numerous Resistance members to fight Chrom's army and aid Pheros to her disgust, but in the end, it was futile and the fort was seized. Later at the Demon's Ingle, Excellus spoke to Yen'fay before his battle with his sister and Chrom's army and openly mocked him. After Yen'fay's death, Excellus revealed to Say'ri why Yen'fay allied with Walhart, and laughed as Say'ri learned the heartbreaking truth before warping away.
Believing to still have much of Chon'sin under his control, Excellus returns to Valm to prepare for the battle between Walhart and Chrom. After Chrom defeats Walhart outside of the castle, Excellus looks outside after Walhart tells him to. He is surprised that the Chon'sin forces are now allied with Chrom and his army, unable to understand why they had turned sides. Walhart deduces that they were loyal to Yen'fay and not under the control of Excellus' blackmail. Walhart also reveals Excellus' betrayal towards him, having learned about his secret deal with Aversa. Excellus attempts to flee the castle, but Walhart threatens to kill him on the spot if he tries to. With no other choice, he fights against the Shepherds, only to be killed in battle.
Excellus' gender is rather confusing, due to his ambiguous appearance and his use of feminine titles. A conversation between Aversa and the Avatar in Chapter 22 implies Excellus is a eunuch.