In JTA none of the characters are 'weak' in the common sense, They are well above average and pretty good at what they do... However they have weaknesses.
Azreth is incapable of using magic due to his own stubborness as well as some attention deficit.
He undergoes consistent training with Vernon to get past his limitations so he gets the hurdles he himself placed out of the way so that he can grow his healing magic, his personality is also a problem at the start as he is very much a 'action guy' and the idea of having a magic that helps others is not too fun for him. But he finds out he can heal people and still be a badass about it, so it's all good.
He also has some seriously unresolved issues with his dragon heritage that he learns how to get over as the series goes on, eventually he becomes an interesting mixture. Being a dragon that is resilient, strong and capable of healing.
Grey starts off with most of the knowhow he needs to be a badass, and is without a doubt one of the best fighters of the world, as well as deadly sharpshooter.
His weakness comes from being 'set in his ways' having been a bounty hunter for decades he is used to taking down opponenets that are not on his level, that and his underdevolped emotional side make it harder for him to get a good fight going as well as to trust his newfound allies,his insticts were honed for something else entirely, ambushing easy prey is often different from actively seeking out tough enemies.
He is also a 'batman-esque' character so his growth happens with equipment, pretty much anyone he fights ONCE doesn't stand a chance the second time around, he has a lot of contingencies and equipment that helps him topple even the biggest foes... Making his lack of magic powers almost a non-defect.
Vernon is the one that I would call conventionally weak, his physical prowess is minimal, he is dexterous but not strong and often gets bashed around by bigger enemies.
He does possess a massively powerful magic, that even with it's great costs would allow him to dispatch powerful foes, however here comes Vernon's weakness... He is a complete pacifist... He constantly doubts himself, and feels for his opponent which prevents him from ever going all out.
A lot of his training is done by Grey, he teaches Vernon how to dissentize to those things, and how not to worry too much about hurting people that are literally doing all they can to kill him.... Slowly he gets over that hurdle, but in a different way as well.
In order to maintain his pacifist ways, he learns the sword and dedicates himself to completely mastering it like no other person ever did... Basically he uses a blade so he doesn't have to outright kill anyone with his magic, essentially making it the 'less of two evils' he could inflict on you
His control over his magic is pretty damn good, so as he learns to get past those mental hurdles he also becomes more confident in using it to fend off foes and slowly he develops a rather unique 'sword and sorcery' combat style. His magic is still damn lethal if applied directly to a person, but thanks to his own ingenuity and some gear Grey makes for him he slowly turns into the hero is meant to be ^^
