"The Red Mage commands not only white and black magicks, but also those uniquely her own."
Only Viera can become Red Mages. Red Mages are very versatile, as they have good Magic and Strength. They are very good mages as they can use Black, White, and Green Magic, as well as the ability Doublecast that can be used not only on Red Magic, but on any other magic (recommended Summoner) as long as you have them as the secondary A-ability. They can also be great physical hitters too, so one needs not worry much if someone casts Silence on them.
One of the most potent uses of Doublecast is to combine it with the Blood Price support ability, which allows one to cast magic by using double its cost in HP instead of MP, so she will be able to use Doublecast at the beginning of the match. As long as the Red Mage uses its second cast to heal herself, she can easily negate the HP cost, essentially getting free magick.