I checked the comments to see what this story is about (which is about him having his stepsister as his ex but I wanted to know more about their personalities) but I couldn't find a single one, most of them are just guessing even before reading the manga.
The two MCs are a bit bland, they don't overreact even a bit for comedic purposes, they're pretty run-of-the-mill kind of characters.
First of all, the male MC is a bit of a teaser, he teases the girl a few times, and in their past relationship, he used to be a kind sweet boyfriend, but he became a petty moron for some stupid reason.
The female MC is a girl trying to act mature. She was a sweet, shy girl in their past relationship (she even felt excited when the MC invited her to his room in the past but felt disappointed when he didn't push her down) that ended up getting friendlier with others.
Their relationship would not have broken up if the MC wasn't such a moron, getting annoyed that the female MC started having other friends than him. Then the female MC got annoyed that he apparently had female friends, then both of them waited each other to contact, but they both didn't even try to contact each other.
This manga has a bit of cuteness, but it doesn't really have that comedic vibe, none of that funny fun fun stuff and all.
With their stale character personalities, the author will likely add in some rivals or extra characters to somehow make the gray manga a little more colorful.
Or he'll add more Ecchi content.
They are a bit sweet, no doubt, but their characters are pretty bland