Mortlach

is a Holy Knight serving King Arthur Pendragon of Camelot. He is also the brother-in-law of Ironside and the uncle of Diodra.

Sistana arc
Mortlach appears in the middle of the battle between Ironside and Percival's group. Mortlach stops Ironside from continuing the fight, Ironside demands that he not interfere, but Mortlach reminds him that whoever destroyed the monsters he summoned must be someone as strong as the Seven Deadly Sins, so in the face of such a threat, it is best for them to retreat. After collecting the pieces of the Coffin of Eternal Darkness minus the Dragon Handle that Donny kept, both Holy Knights leave.

Wallnack arc
Mortlach was training Diodra with swords, surprised by his nephew's progress and asking him if he was training alone in secret. Diodra asks him not to tell her father so as not to worry him, to which Mortlach says that Ironside will end up scolding him. Diodra then asks him to continue telling his story, and Mortlach tells him about when he and his sister were punished by his father for sneaking out drinking and gambling, warning the boy not to do the same. Diodra asks him why his father keeps telling him lies that Britannia is nothing more than a world full of dangers, further fueling his curiosity. Mortlach tells him that he understands what he feels, but that her brother-in-law does not lie and that he is sure that he loves him from the bottom of his heart.

When a bird arrives to inform Mortlach of a mission from king Arthur, he leaves, leaving Diodra in charge of Turret and Rothes.

Demon Realm arc
Mortlach and his fellow Holy Knights track down the Four Knights of Apocalypse, finding them when they cross the entrance to the portal into the Demon Realm. Deciding to follow them, they end up being thrown into the void by the relic that connects them to the Demon Realm, saving themselves from the fall by summoning a Purgatory Beast that takes them flying.

Meeting to decide how to proceed, Mortlach notices something strange about Rosebank when he refuses to speak, claiming to have been infected with the Demon Realm's miasma. Confirming with his other companions, Mortlach attacks Rosebank, who ends up revealing herself to be an illusion created by Gawain.

After trapping his companions in her Super Slow spell, Gawain asks Mortlach to tell Arthur to leave her alone. Mortlach tells her that there is still time for her to return and beg his forgiveness, but Gawain removes his helmet with a magic beam, stating that she does not beg anyone.

Mortlach then tells her that despite being "commoners", they all have someone they love and for whom they would give their lives, something that Gawain cannot understand since she never felt love for anyone, not even the father who raised her or his uncle who gave her everything. Gawain reacts annoyed, stating that she never got what she wanted in Camelot, answers to who she is, who gave birth to her, why she can't feel love for anyone and only be interested in women. Mortlach says that she joined the Four Knights because she believed that by fighting alongside those like her, she could find those answers. Gawain rages at his taunts, not noticing the knight who surprises her behind her and knocks her out with one blow.

Mortlach and the rest then happily greet the real Rosebank, relieved that she is safe. Showing her a captured Gawain, Mortlach reminds everyone that they cannot kill her because she is the niece of their king. When the group finds itself unable to do anything but wait for the enemy, Mortlach says that they are in no rush since they already know the enemy and its strength, and that the best way to stop the prophecy is to take out Percival, who is below Tristan and Lancelot.

Trivia

 * Like many of the Camelot Holy Knights, Mortlach is named after the, a distiller of Scotch whisky in Dufftown, Moray, Scotland, founded in 1823.