Tidus and Vaan are young and that is portrayed clearly in their mannerisms and actions. Vaan often makes comments that irritate others and is very dysphemistic in his honesty. Both are quite provincial in that they are reckless and couldn't care less about the day after tomorrow. Both heros accomplish their goals and worry not of what the future holds, oftentimes dealing with the harsh realities of the future the moment they manifest themselves. Although Tidus comes to fall in love with Yuna--a stark difference between him and Vaan--he more or less falls in love with her personality and her geniality. Vaan holds Ashe in highest regards and is very respectful toward her, thus showing his admiration for Ashe as a human being and the way she carries herself. I personally think that Vaan has feelings for Ashe, but is weary to express said feelings due to Ashe's tragic past experience with lost love.
Kimahri is Yuna's guardian and his sole purpose of living in life is to protect Yuna. Basch takes the same role as Kimahri and has stated that he will stop at nothing to realize Ashe's dreams. In both cases, we don't really have much background as to -why- they feel such a close connection to their heroine, but it is obvious to the naked eye that their devotion is indomitable. Next I come to Lulu and Fran. Both are very intelligent and specialize in magic. Fran is connected to nature by strings that lie deep in her heritage, yet Lulu takes the path of black magic and utilizes the elements to bring down even the most ferocious of fiends. Lulu often has wise aphoristic hints and tips to give to the party, and Fran has a keen sense of nethicite that is very important to the group.
Next up is Penelo, who most closely parallels Rikku. Both girls actually look very similar and they play a very minute role in the story itself. Rikku and Penelo are more "support" characters than anything, yet they are still important to the functionality of the party. Both act as the team's mascots and cheer on the group even through the dimmest of times.
Finally, you have Balthier, who is probably the most knowledgable entity in the party. He represents Auron's wisdom and Wakka's youth, in my opinion. He knows more than the rest of the group, yet he is not stern and has a mellower side that corresponds with that of Wakka. Both Balthier and Auron have very intricate and detailed pasts that slowly become unraveled as the story unfolds.
I dunno, that's just my take on the two. Feel free to agree/disagree.