Yep, Trainer. Mascot Rikku at least has the White Mage spells for free, it has the godlike stats typical for Mascots, its Eraser skills come in handy sometimes (albeit VERY rarely) and its ultimate ability is surprisingly accurate. Trainer just goes completely against my preferred style of playing.
Yeah you can put Brother in your active party by selecting Party Members in the menu. Although you're still limited to three party members at a time. You'll see.