He wasn't one of the greatest mages, because he was a commoner and didn't receive training he was only tier 3 when he died. The other heroes protected him for his ability to analyze and seal his opponents magic. So that's why he wants to be in alpha class, so he can become a great mage and save more people. That's why he keeps working on getting the others leveled up before him.
But... He was one of the greatest mages because of his knowledge and his analysis magic even though he was just at tier 3, that's kinda why he was alive for so long...
And since we know that "tiers" have more to do with talents, he wasn't very talented to begin with so the best he can do is find a way to stack mana to simulate the power of higher tiers(something more than the jewels on their uniforms)
I think the overall story line is kinda slow paced.
We havent seen anything yet related to his past and only now after 110+ chapters he got a power up