Katie starts to get close to Carter because in her own way, she's trying to protect her friend, Eric. She wants to keep him away from Quinn so Eric will be happy.

The more time Katie spends with Carter, the more drawn she is to him and suddenly she finds herself falling for him. Really, truly falling for him. Carter finally gets the woman ready and willing to put him first.

Quinn sees how close the two are getting and tries to sex up Carter again, but having realized he wants more he turns her down.
Bill, angry about losing Katie to Carter, tries to pull out all the stops to "win her back", but Katie turns him down in favor of a connection that's more natural and without the baggage of Bill and Brooke.

Having turned down Bill and Quinn, Carter/Katie express how they feel for one another, dive into a slow burn of a relationship and in time Katie introduces Carter and Will, and Will enjoys the company of his new "friend", Carter and how happy his mom is around him.

Eventually with some bumps along the way, the two get married and adopt a child (because of Katie's heart issue) and Carter has the family he's wanted.

Bonus: In order to stop what's budding between Carter and Katie, Bill and Quinn team up and end up getting steamy. Quinn realizes she wants more than what Eric gives her and leaves him - for fun with Bill, which drives Eric, Brooke AND Ridge crazy!