So since he was a child he had a crest but couldn't use it properly. He then waited for 15 years before evaluating?! Do you wait for 15 years before going to the doctor when you can't lift your arm?
Man this guy is beyond a dumbass. He was told he had high magical capabilities and yet he never thought to test his own aptitude toward what skills would benefit him the most. So he gets a mage crest and can't do shit and for 4 years he acts as a burden to every party that he joined. And not once did he look into why he couldn't use his crest properly.