Yep. Hero is a boss class in this hack. I think the best gains I saw out of any promotion, and incredible caps too.
Gerik is another candidate for that Master Seal in my opinion since having another Hero class is broken. He won't grow to cap resistance like Garcia does, but he will cap the rest of the stats rather nicely such as skl, def, str, hp, and speed. I am sort of regretting having made Ross go pirate -> warrior. I would like to have seen how useful he would be as a hero.
Natasha will not be extremely useful, even against monsters since they have so much damn HP. Lute is more useful in my opinion in the long run. Having 30 magic means she can use those range staves better than Natasha can. You'll eventually run into a secret shop which allows you to use AirCalibur (super useful) as anima magic. I wouldn't bother wasting the master seal on Tana since you'll get a Pegasus whip from Cormag on chapter 13. I did not use him though to see how well he would turn out.
Tana will cap everything really nicely, but her low con was a huge pain. When enemies have capped defence her using an iron lance at 24str cap isn't doing a whole lot of damage. Equipping a steel lance damages her speed so much she cannot even double anymore.
Near the middle and end, Garcia, Gerik, Ewan, and Amelia made up the largest part of my frontline attack force. Besides, Tana cannot wield halberd or hammer either. Again, I say use the master seal on Gerik. I think you'll get the most use out of it. But the next two chapters are a breeze anyway since you'll face a ton of unpromoted monsters which are not as tough as those generals, sages, and paladins. Also, make sure to get the Halberd drop by making the centaur boss to equip his bow.