Nanki-Poo subsequently fled the court to escape marriage with Katisha and assumed the disguise of a traveling musician. In Titipu he fell in love with the beautiful Yum-Yum but left her when he discovered she was engaged to Ko-Ko (her guardian and cheap tailor). Later Nanki-Poo learns that Ko-Ko has been condemned to death for flirting and hurries back to Titipu to claim Yum-Yum. He now learns that Ko-Ko has been promoted to Lord High Executioner, thus preventing his own sentence of death from being carried out. In fact, Ko-Ko is to marry Yum-Yum that afternoon! Through some clever conniving, love triumphs and Yum-Yum and Nanki-Poo are betrothed. The village celebration is interrupted by the relentless Katisha who rains blight on their festivities and claims Nanki-Poo as her own. What follows is the most delicious and hilarious romp through topsy-turveydom that Gilbert ever conceived. To read the opera, google for Mikado Libretto.
As a character, Katisha is probably the most interesting of all the G&S contralto roles, certainly one of the most demanding both vocally and artistically. The challenge in the role is to allow the audience to hate this miserable character who comes between two young lovers but later let the audience discover the soft underbelly of this beast and gain its empathy. And finally Katisha must bring the audience a full 180 degrees so they may rejoice with her as she gains happiness for herself.
Katisha doesn't appear in the operetta until the middle of the Act One Finale -- but a great entrance she makes! In the middle of her pleadings for Nanki-Poo she sings a beautiful succinct aria, "The hour of gladness is dead and gone."
In Act Two Katisha gets some great action when she returns to Titipu with the Mikado himself. She gets to sing a couple of duets and a quintet, and her gut-wrenching aria "Alone and yet alive," probably the best contralto solo in all the G&S canon.