The Sin of Sloth is often misunderstood. The traditional meaning wasn’t lazy, rather a lack of drive. Monks or religious figures who lost their faith or became indifferent.
In this sense, Ace embodies sloth. It’s not so much that he doesn’t care about the people closest to him, but remains rather apathetic to the things that go on around him. This changes gradually as his siblings find love and the group dynamic evolves. Not so much because Ace cares—he’s very much a “to each their own” sort of guy. But when the expanding Sin family nearly ends the world, Ace’s apathy hardens into something closer to resentment.
Ace thrives on the status quo—his own status quo. But when his worldview is challenged and he’s given something to care about, he finds that empathy is something he can’t just shut off, no matter how hard he tries.
*denotes the book in which Ace is the lead character