I was indifferent towards Vivi for a good portion of the game. Although visually he's the ideal mage (albeit quite cute), his MP was balls for a majority of the game until you get the Half MP ability. A couple of -aga spells would set you up for needing an Ether (which admittedly I'm quite reserved with), so I'd spend most of the battle just using Focus until casting was actually worth it. His developing maturity was quite surprising for a 9 year old too.