Atsuko

Atsuko is a woman whom Genma claims he once dated when he was younger, in the anime episode Akane's Lost Love... These Things Happen, You Know. A beautiful woman with long, flowing brown hair, Genma claims that he bluntly dumped her for another girl, telling Atsuko to her face that the new girl is much better than she is, being a better dresser, more beautiful in general, kind, sweet, innocent and from a very wealthy family. For this affront, Atsuko beat Genma senseless with an oversized mallet.

It's unclear if Atsuko actually exists, or if Genma simply made her up wholesale as part of a poorly devised plan to try and jar Ranma into treating Akane better, as their relationship was particularly rocky during those first few days.