You all need to read the manga... sigh.
Spoiler warning, here we go again.
There is no promised girl, ok? Rather, there is no ONE promised girl. You find out as the manga goes further along that Naru, Keitaro, and Mutsumi played together a lot as children as a trio. Keitaro made many promises.Keitaro is the one who made the statement about two people going to Toudai and finding happiness, not either of the girls. He said it to both of them. According to the manga, Naru and Mutsumi both had kiddish crushes on Keitaro, but Mutsumi, even at that young age, accepted things and told Naru that Kei-kun could be her boyfriend, and that she didn't mind. In other words, she supported Keitaro and Naru being together, even at that young age... which again, was kind of silly since they were all so young anyway, especially Naru.
So what does this mean overall? Not a whole lot. The promise drove the show along early on, but as the show progressed, you discover that the promise of long ago is not nearly as important as present day feelings. I think that's the idea behind all of that. Even if there had been a promised girl, and even is it were Mutsumi, you know Keitaro would have still chosen Naru. That's just how Keitaro felt.
