I'm a huge apologist for Love Hina because the anime never did the series justice and the manga was brilliantly written, but it wouldn't be nearly as good without Mutsumi Otohime.

Mutsumi started as the character that could've been Keitaro's promised girl being as she was also present at the beginning of the series when Keitaro made the promise.
Mutsumi also failed her exams multiple times, even worse than Keitaro did but her bad health and luck was always her undoing as opposed to a lack of intellect, in fact about half way through the manga she performed her most genius act in getting Naru and Keitaro together.
At her home town in Okinawa during a visit she loses her memory and she's made to believe that she's a child and playing with Keitaro as his girlfriend, this comes to a zenith when she ends up staging an actual wedding and is about to marry Keitaro only for Naru to call out to stop it, it's then that it's heavily implied that she faked her memory loss all along and did it to bring Keitaro and Naru together, succeeding in what is argubly one of the most genius acts in the anime franchise's run.
While I never really liked her role as the ill girl that always collapsed, a joke that wore out it's welcome pretty quick, her other traits more than made up for it.