@Meybi, it could be from redice side, some kind of shitty editor wanting to add things, or even the novelist himself because he wants some eyecandy to an otherwise a story about a single dude going around finding clues about the past and future, turning it into a story about a dude who goes around hitting hot milf around his area and got cockblocked by his trusty, cute, and disgustingly annoying sidekick.
or it could be the artist personal request? "how bout adding some cute sidekick for the MC, it'll make the story more refreshing aye. Oh anyway, I love nagging female characters that absolutely adds nothing to the story as long as they're cute, I can add her right?? This Isabelle from the novel beginning seems to have a lot of potential, i can use her, right?".
In the novel isabelle is just a passing character, completely irrelevant as of the current translation, and also, the MC isn't a lecher that will try to hit at every hot aunties near his area.