I have to disagree with josephgomes619's opinion on one point:
"Author tries to force a harem despite the FMC being the only possible girlfriend since Ch 1."
The author does not force a harem, surprisingly. MC only has eyes for the main girl, a kuudere (mistaken for a tsundere for good reasons, but she's been nothing but ice, not thorn). He gave up at one point submitted to girl #2 and tried dating the second girl only to realize his mistake and is once again keeping her in her crosshairs. She's admitted to being friends with him, but it's only the latest chapters she seems to be becoming aware of him as something more. Whether she acts on it is something else. She's only admitted to showing respect.
Girl #2 is just aggressive/assertive (the opposite of FMC) and is trying to win him over, knowing the full situation, and refuses to give up. They've done some lovey-dovey stuff, but only because she pushed it along. She actually helps move the story along. It's unlikely, but given her personality
Girl #3 is cute, but nobody knows what she's thinking. She's a harem possibility, but we'll see. SO really, it's a harem, but not a harem.
So really, if the author was going to force a harem, the MC would have accepted all three and FMC would be all over him instead of "whatever" all the time.
Otherwise character development is present, it's just slow, so you need to have patience. The MC will definitely annoy quite a few people here, but credit where it's due: He's fully aware of his shortcomings and chastises himself on behalf of the readers. Latest chapters have had him manning up and helping main girl out, but again: slow progress.
If you're looking for a quick read, this is not the manga for you. The story is meant to be enjoyed on the low and slow. It's borderline a Slice of Life, but there IS a story. This is just one of those rare stories where the plot is stretched out over time instead of in arcs.
To be clear, I'm not knocking him for dropping this. No one should be forced to read something they don't like and props to them for sticking out 100 chapters. They really tried! I have a rule that every reader should at least try for the first three chapters to decide if it's worth continuing or dropping. 3 chapters is more than sufficient progress to determine whether you'll like the story, characters, etc. But it's nobody's place, not even his, to tell others what to read or what to drop. Giving your opinion and feedback is all well and good, but only the reader should decide if it's worth picking up.
Very sorry, didn't mean for this to be a wall of text. Point is, judge for yourself, don't let others decide for you.