You can tell how weak characters are in this manga when:
One - I can't even tell any of them apart, every fucking one of them including the main couple can be divided in, cheery light hair girl and somewhat akward dark haired girl.
Two - The author can't develop any character on their own so she has to manifest some curveball drama out of the depths of hell to make anything "interesting" happen in this mess of a manga.
Creating a new character to push the story foward isn't necessarily a bad move, afterall it happens all the damn time in media, but it is meant to be just A way to move the plot foward, not the ONLY way, if you can't move your narrative foward without constantly introducing new elements you fucked up.