Could a novel reader let me know why Callisto's heart meter went up so slowly? It's clear ...

Unfortunately I don't remember that ever being elaborated in the novel.

My personal theory, though, is that he is a very guarded person who is slow to trust. He was clearly interested in her but it took him a while to realize his own feelings, which is why I think his meter was only at 70% when he first proposed.

But that's just my interpretation, we'll never really know... ╮(￣▽￣)╭

so my guess based on what ive read, is that his meter was the only one representing real romantic love. the others all had either familial or some kind of twisted "love" meter or whatnot, and he's the only one who genuinely started to fall in love with her. so in the novel when she asked him later about that time when she asked if he loved her at the party, and his meter was only at like 80 something, he was already in love but didnt understand his feelings because they're both ppl who didnt really believe in love.

Contains spoilers.

it's been a while since i finished the novel but, uh theres a couple points to this;

1. I remember eclipse's meter and callisto's affected each other when there was sudden spikes. so when pen got too close too eclipse, that made calli jealous with his meter going down.

2. as rantpanda said, all the other characters have twisted affection. in this sense, all of the charas's affection are correlated to Pen's past / game setting. Pinkie bro's is guilt for stealing yvonne's necklace and framing pen. Derrick's guilt for losing yvvone in the festival as a kid. The duke's guilt for overlooking pinkie's action, and trying to compensate her monetraily, and covering up her bad rep, than thru affection. eclipse - affection nurtured into possessive obsession.

None of these qualitfy as "love" vs callisto who is the only who has romantic affection and the closest thing to love for her.

also i think the 100 on the affection meter doesn't satisfy till the love interest either names, or acknowledges it as "love"

3. Both penelope and callisto supressing their emotions. pen doesn't have time or ease to leisurely take her time and enjoy life with calli in a situationship. She cant rely on an affection that would go away. That wont get her out of duke's house, or help her survive, or fulfill the system's conditions. That's why she made it clear to calli she needs someone who "loves her" to get her out of the system's hell (he'll find out about the system later too).

i dont remember if calli mentioned this in the manhwa, but basically he didnt want the royal family to control pen and take away her freedom and choice. Given his past with his family, he doesnt feel like he has the capability of "loving" normally, is my take on it, which he lowkey got over, after seeing pen in a comahere.