Location: Besaid Temple.
Battle style: he is a flyer, so is able to dodge most attacks. His Sonic Wings also delay enemies, so he will have plenty of turns.
Overdrive: Energy Ray and Energy Blast(non-elemental). If Yuna has her Celestial Weapon (Nirvana), he will do over 9999 HP damage.
Location: Kilika Temple.
Battle style: he has the element of fire at his side, with which he can also heal himself.
Overdrive: Helfire (fire). If Wakka has his Celestial Weapon (World Champion), he will do over 9999 HP damage.
Location: Djose Temple.
Battle style: he has the element of lightning at his side, with which he can also heal himself.
Overdrive: Thor's Hammer (lightning). If Kimahri has his Celestial Weapon (Spirit Lance), he will do over 9999 HP damage.
Location: Macalania Temple.
Battle style: she has the element of ice at her side, with which she can also heal herself.
Overdrive: Diamond Dust (ice). If Lulu has her Celestial Weapon (Onion Knight), she will do over 9999 HP damage.
Location: Bevelle Temple.
Battle style: he has the most powerfull attacks at the time you get him. Notice that he has very powerfull black magic spells at his side.
Overdrive: Mega Flare (non-elemental).
Location: Cavern Of The Stolen Fayth.
Battle style: You have to pay him 250,000 Gil in order to get him to fight with you. Notice that he has a couple of strong attacks which you can't control.
Overdrive: Daigoro, Kozuka, Wakizashi and Zanmato (non-elemental).
Location: Baaj Temple.
Battle style: He can only use his 'Pain' attack. But it inflicts a lot of damage, and sometimes instant death.
Overdrive: Oblivion (non-elemental).
Location: Remien Temple.
Battle style: They may have some strange attacks and all, but are very powerfull.
Overdrive: Delta Attack (non-elemental).