Merlin is first seen in Grandeur, when the party visit the Don's house. He doesn't get recognized by the party, and actually covers for Lazarus who's hiding under the couch.
Merlin reappears in Part 2 as the party is infiltrating Grangold. To finalize the preparation of their plan to infiltrate the castle, Luka and Mephisto must meet the Don of the mafia who is preparing the final steps. Luka and Sonya are surprised to discover that said Don is none other than Lazarus himself with Merlin, while Mephisto herself is surprised to learn that the hero is related to the boss of the mafia. Lazarus explains his plan to bomb out empty parts of the city to attract attention while the group infiltrate the castle through the sewers. He proceeds to join the group just before they enter the Underground Waterway.
Unfortunately, just before the group enter the castle, a brainwashed King of Grangold attacks them and transforms into his proto-human form. Merlin and Lazarus decide to hold him back while the hero's party keep progressing. They manage to hold him back for a time, but suffer heavy wounds and are about to be killed when Tamamo steps in and saves them.
They are later seen at the Banquet organized to celebrate the end of the war, where they're split between scolding Luka for getting himself involved in overthrowing a government, and running away from Sonya who's angry at them for hiding their involvement in the mafia for so long.
They are later seen at the tavern in Gold Port where they can be recruited once and for all. They proceed to set up a casino in the Pocket Castle where Luka can earn some additional equipment and the Underworld License, the job item required for the Godfather job, as well as a few underworld-related jobs.
He is also seen at Upcoming Events discussing with Lazarus about a grave that was being referred to the unfinished letter that Lazarus was trying to hide from Luka.
Merlin: “Would you like some tea? Or do you have a job for me...?”
Lazarus: “We've gotten completely roped into their trouble... I never imagined we'd get stuck tagging along on an adventure again.”
Merlin: “You say that, but you're having fun aren't you? Your heart is still there from when you left on an adventure with Marcellus.”
Lazarus: “Bullshit...I'm not having fun. I'm just worried about those damn kids.”
Merlin: “You're like an eternal child with that stubborn resilience of yours...”
Lazarus: “Even adults can be stubborn. I know you're determined to have fun, old man.”
Merlin: “Yes... because I am young at heart.”
Lazarus: “Jeez...you old geezer...”
Sonya: “When you joined up with Uncle Lazarus, you weren't doing anything bad were you...?”
Merlin: “To rule over evil is not necessarily evil in itself. Rather, you should think of it as restraining and controlling evil?”
Sonya: “That's sophistry, isn't it?”
Merlin: “Yes, you are right.”
Sonya: “Don't admit it so readily... An old man can't support himself.”
Merlin: “I represent the Don. I won't be troubled so easily with surviving.”
With King of San Ilia:
King of San Ilia: “Merlin... The Don Father of the table. You aren't confined to just being the shadow commander, but also the Don's confidant.”
Merlin: “Let's stop trying to dig out the root of evil in a place like this. We cannot afford to inconvenience Mr. Luka's party.”
King of San Ilia: “I'll be sure to drag you out before the law... ...but here, I would like to be your companion.”
Merlin: “Then let's refrain from mixing public and private conflicts with one another. Shall we defer the root of evil to the halls of justice then?”
King of San Ilia: “............”
Merlin: “It's not much, but I happen to have some experience in painting, too...”
Merlin is painting a picture...
Merlin: “Looking back, we've come quite far indeed...”
Merlin is deep in thought...
[Random enemy takes damage]
Merlin: “It's not much, but I happen to have some experience in playing music, too...”
Merlin plays the violin...
What a painful sound!
[Chance to Confuse everyone]
Merlin: “I've prepared an afternoon snack...”
Merlin presents a gift!