Despite being a quiet person, Jewel is extremely passionate. He seems to wall himself off from the people around him in an effort to control the strength of his emotions. This is demonstrated by the callous way he seems to act towards his mother when she is alive, which contrasts with the devotion he shows to her in death, including giving up his own horse for the mules needed to finish transporting her and rushing into the river to save the coffin.
Jewel is defined by action. He doesn't talk much as he does not value language. Like his mother, he believes language to get in the way of what you truely mean--actions speak louder than words. Neither of them trust words.
Read the character analysis that I have attached a link to below. You won't find a better description of this complex person. By saying that Jewel is reticent, the critic means that Jewel rarely talks.