“Maybe Diluc finds out when he's away from Mond, travelling the world to find out more about the Fatui, and he's just. Taken aback. Because the person in the picture can't be his parent. Their eyes are cold and lifeless, and they seem a completely different person from how they are now.”
Oh man, now that has me thinking about just how much Crepus and the family darling had with him truly changed them for the better. I’d suppose that before meeting Crepus, darling embodied (if not set the bar for) every negative stereotype associated with the Harbingers—civil and polite but insincere at best; cruel, cold-blooded, and maybe even bloodthirsty at worst. Upon first meeting Crepus, darling obviously put up a courteous front, but found that the longer they talked the more Crepus seemed to draw out darling’s true self—the more human, casual side of them that they probably didn’t even realize existed.
Over time, everyone around darling begins to notice the shift in their personality. From being cutthroat and and ruthless in their debt collecting, they begin extending the deadlines and even pardoning those unable to pay for their loans. They begin to be more friendly and relaxed with their subordinates. They butt heads with the other Harbingers over how to deal with high-ranking Fatui targets, and at times (much to the other Harbingers’ surprise and chagrin) suggest sparing their lives and giving them mercy.
The most apparent change in darling is physical, and it’s of the light in their eyes. One of darling’s core attributes as a Harbinger was the utter lifelessness in their eyes—complementary to the amount of death and carnage they caused as part of their duties in the Fatui. Little by little, and coincidentally every time they met with Crepus, a little flicker of light would appear in their eyes—not enough to be apparent at first, but would gradually grow bigger and brighter over time.
While darling’s departure has frequently been told as a drunken lament by other Fatui members who worked with them, none of them could ever deny the happiness darling practically beamed with upon being let go by the Tsaritsa. How they seemed to walk on air as they walked down the halls of Zapolyarny Palace for the last time, embracing as many of their subordinates as they could and wishing them all the best.
Over at the Dawn Winery, you could probably catch Adelinde reminiscing at the memory of darling (freshly out of the Fatui) embracing Crepus with a love and warmth felt by everyone in the household—a love and warmth that would continue to bloom as darling settled down with the Ragnvindrs as Crepus’s partner and parent to Diluc and Kaeya, and a love and warmth that still lingers every time darling comes to visit their two sons.
I'm an absolute sucker for tropes like this, and seeing all of this absolutely wrecked me.
The Reader's personality changing, the sharpened edges being smoothed out, and all because they've met someone who brings out the best in them, who sees them as more than just a weapon.
Maybe the Reader becomes aware of how much they've changed (or made aware of the extent of that change) when they save Crepus from enemies of their own volition. They weren't asked to by anyone, they just stepped in and did it because they found themselves gripped with fear at the thought of losing him (and maybe that's also when they realise that they might like him a lot more than they've previously thought). I also like the idea of the Reader telling Crepus to avert his eyes right before they start fighting, because they don't want him to see them as an instrument of war (and they aren't sure if he can handle seeing it), and they're surprised when they see that he's been watching their every movement, eyes wide and lips parted. The Reader expects to be pushed away and shunned, but to their surprise, he thanks them for saving his life with such soft sincerity that Reader can feel their heart ache.