I might not sound like it but I have quite a soft spot for Yuna. To the contrary of Lenne, Yuna is not exactly someone I'd like to be myself, though I find her character interesting enough to keep an eye on. Like Auron once said, "Yuna is easy to read but hard to protect." You know what she's thinking, but you can't tell how she'll act based on her thoughts. She's not a bad instance to observe at all, really.
Despite the whole "cameo" talk that's been going on in FFX-2 fan communities for years, I don't think Yuna's like Lenne at all whether it be looks or personality. However, there is one trait they share deep down: their willingness to sacrifice themselves for the greater good. This probably comes from their profession as summoners, but also the fact that both are kind hearted individuals to the core. They're equally good at causing their men extra headaches, too.
Interested in more cameo-centric theories? Time to move on and never look back.