Way #1: At Cape Howl, after defeating Karsh, Solt, and Peppor, allow her to join. *NOTE* If you do this, you will not be able to recruit Leena. If you recruited Poshul before going to Opassa Beach,
you can have him and Kid. If not, he's lost for the rest of the game, so wait until New Game+.

Way #2: When Lynx stabbs Kid at Viper Manor, either save her to recruit her quicker, or follow way #3.

Way #3: Don't save Kid and after Hermit's Hideaway and Fargo's S.S. Invinsible, go back to Guldove. There you will probably recruit Doc, and no matter if you like it or not, she WILL join.

Way #4: After Kid is put to sleep in a coma, go to Divine Dragon Falls with the Tears of Love and Hate and put them on the pedastals to make the long lost 7'th mystical Element, the Chrono Cross! Make sure you have the Mastermune. Then, go to Hermit's Hideaway. "Talk" to Kid. You will go into her dream. I'm not going to tell you anything, so you may want to follow a FAQ. Afterwards, when you see Lynx, tell Kid the nice choices to say. After that, she will think of you as, charming, and, JOIN!

Way #5: Get her off the "Use the Chrono Cross at Sprigg's" cheat.