Tyrell was born seven years ago, when he awoke in the care of clerics of Tyr. The clerics told of how he was found so near death on the side of the road and brought to them, as one of the few markings unbloodied was the symbol of Tyr on his shield. Not long after he gained consciousness, the clerics discovered that Tyrell suffered from amnesia, forgetting who he was and where he came from. The holy men determined from his armor and clothes that he was most likely a paladin in the service of Tyr, and thus named him Tyrell, meaning "son of Tyr" in an old, forgotten language.
Once Tyrell was nursed back to health, he vowed to travel the world and find his past. Most important was to determine the events that lead to his near death. While his history was lost to him, he still retained the skills and the conviction that convinced him that the clerics were right, that he was a paladin of Tyr.
Tyrell has formed two hypotheses regarding his brush with death. The first is that he was fighting a duel or otherwise championing the cause of righteousness, but had failed; the amnesia is then punishment from Tyr for his failure to uphold the war for Good, and that his penance is to redeem himself by growing to be better than ever, and to right the wrong where he had previously failed.
The second theory is that he was waylaid by numbers beyond his means, and though left for dead, was saved by Tyr; even though his memories were lost, the touch of Tyr left (and amplified) his devotion to his god is his mind. In this scenario, Tyrell feels he must find his attackers and defeat them honorably, both in his name and the name of Tyr.
Regardless of the cause of his injuries, Tyrell is dedicated, perhaps more than before, to the cause of Tyr. While not as devout as a true cleric, he is very strict to his beliefs. He does, however, realize that zealotry can be an unattractive attribute, and makes an effort to keep his convictions to himself.
His current companions suit his needs well; they are adventurous, brave, and powerful. He feels they can help him on his quest, and since they are aware of his past -- what there is to know -- and his quest, he does not pressure them about it. The main reason for this is because he has no clues or evidence to follow, and thus nowhere to steer the party. It is his belief that through faithful subservience, Tyr will guide him to resolution.