As a newcomer to this site, I notice postings and threads from time to time about Kay Adams, which frankly, caused me to re-evaluate her character.
She really is an ungrateful, shrill b*tch when you think about it. Michael (if not the plot) would have been better off with Apollonia as his wife, and you gotta read the book "Blink" to understand the impact of his being hit by a thunderbolt. Michael was really "thin slicing" when he proposed to her.

Kay should have known exactly what she was getting into, and she could never have seriously believed that the Corleone family would be legitimate in five years. When they get together Michael plainly tells her it is a condition of their marriage that she is NEVER to discuss the family business, so what does Kay do? Just as soon as Michael whacks the heads of the other five families and resotres the Corleone family -- at the moment of his triumph, she comes in and asks him about the family business.

Next thing they move to Nevada (helloooooo did anyone with a brain think Vegas was on the up and up in 1958?) and here is poor Michael trying to go head to head with a US Senator over a casino operation, and what does Kay do when he gets to relax that night? She complains that the family has not become legitimate (and she gets her dates mixed up in the process). Then she retaliates against Michael when she learns she can't take her kids to New Hampshire by going off an having an abortion, which she tells Mike about immediately after he has gone through the stress of the Senate hearings. My God, no wonder the poor bastard was a nervous wreck having diabetic strokes in GFIII. With support like that who needs enemies? Somehow between GFII and GFIII Michael who previously told Kay he would use all his power to keep her away from her children actually gives her custody because it is in the children's best interest. Then she goes off and marries some wimpy guy who she drags to the ceremony in New York, which she uses as an opportunity not only to convince Michael that Tony should not have to finish law school and be an opera singer, but also to tell Michael how she liked him better when he was a common "mafia hood," and how she dreads him. Then of course in Sicily while Mike is showing her around she is making snide comments about the puppet show, about his marriage to Appolonia, and then she sits there at the table and tells him she really DOES love him. Yeesh!