Harem is truly the death of good manga. Seriously, if you're going to do a harem, do it from the start so people know not to bother with it. Don't start with an interesting premise for a romance and slowly introduce new love interests, who everyone knows have no shot of winning, and therefore won't be invested it. The only way I see this being salvaged is if this new girl already has a boyfriend, or if they introduce another male character to balance out the cast.