All the skills and items in the world can't help you if your characters can be killed by a strong breeze. To survive, they need to toughen up. Weapons and armor are a big step, but eventually your characters will have to fight and level up.
Levels are the thing you're probably looking at when it comes to determining character strength. The higher, the better. One thing to keep in mind is that all characters level up at the exact same rate. This means you can usually predict who else has or will level up depending on who has leveled up beforehand.
Not all characters level equally. Each character has different levels of stat growth based around their Class Category. While there are slight variances between individual characters, they play out in the same way per category. They are listed below from A to E, with A being the best and E begin the worst.
If you want to see what the characters are like once they maxed out their level, either for determine stat seed distribution or just curiosity, the stats are right here. This table assumes all stat growth is just leveling up, without any Timed Events, dating, stat seeds or training.