Backstory
Growing up in the original Pandora's box with the other Miseries, Lethe practically submitted to the fate that lied beyond them; to be resealed if freed, to cause trouble if the chance came, to really just live a looping life. He watched the others differ in aspirations or decide to work together, but he preferred to be alone, certain he'd still come across the others in the future, unless one by one he'd come to forget them all. When freedom came, escaping from the box, he idled around and wandered more than anything.
Just like the more level headed miseries, he wasn't really looking to be known or pick fights with humans, indifferent to tormenting them individually even if he occasionally did so. Quickly becoming bored of the human world, he spent a lot of time napping without a care, still uncaring when he one day woke up sealed away. Sleeping in plain sight of a Pandora, he'd been recaptured and sealed away.
Sometimes he did come across the other Miseries when they'd free him. Zerrin would tell him to be more careful only to find him sealed away the next day for being so careless. Scena would stumble upon him sealed and call him a fool, only to never be seen again after being sealed herself. Lethe didn't really care either way, to what happened to his fellow Miseries, or that they didn't care much about him if he kept forgetting who they were.
He really didn't have any complaints about the life he led, at least playing the harp in the realm he was sealed in to pass time. Considering time to go in a circle, he anticipated being freed again, being sealed again, but maybe there would come a day where humans wouldn't consider them such threats. If he kept to himself, perhaps it'd be recognized one day, allowing him to idle around without an interruption.
At this point, he'd continued to lounge around until he was searched for for the first time. The other Miseries disregarded him for the most part, but when Ephialtes and Chimare found the music box he was currently sealed in, they asked for his help in subduing Merikh and Pagule in the future. He had no fighting experience or a care for the world itself, but if his only options were to prevent chaos or join the hassle of enacting it with Ephialtes' enemies, he'd rather pick the more relaxed team than the chaotic one.
Assigned with sabotaging any other groups trying to get their hands on the Miseries, Lethe investigated Merikh and Pagule's whereabouts, and occasionally ran into Miseries on opposing teams. To be honest, none of them want to be in the spot they're each in, still considering each other siblings aside from Merikh and Pagule who are in their own category. Lethe has no ill will towards the others, in fact still not remembering them well by habit, but he assumes they're also searching for a world without so much trouble in the future.