For a very long time, Sister Brighid simply thought the world was full of happy, helpful people. She knew there was evil in the world, but it seemed very far away. When she was assigned to the Oak Grove Convent in Paragon City, she knew she would see the best (and worst) humanity had to offer. Eastgate was a lovely place, but every garden had it's snake.
It wasn't until the mages attacked the convent one night that she got a glimpse of true evil. Sister Brighid did all she could to help the others escape, knowing the Angels were certainly watching over that night; it always seemed the guards were looking the other way or otherwise distracted whenever Sister Brighid prayed for help.
As the others fled to safety, Sister Brighid realized no one had seen Mother Cailly. Sneaking from nook to cranny, she made her way to the sacristy. If the attackers were after anything here, it would be where the sacred vessels were kept, and Mother Cailly would be trying to protect them.
Sister Brighid arrived just after the Heroes of Paragon did. When Mother Cailly refused to flee and leave the relics behind, Sister Brighid did her best to keep them and Mother Cailly safe While the Heroes did their best to defeat the mages ransacking the convent.
When all was done, Sister Brighid was secretly thrilled (but embarrassed) to meet such famous people. They were kind enough to even thank her for her help.
After that, life in the convent returned to normal (or as "normal" as anything can be in Paragon City), but the world seemed to have taken on new colors for Sister Brighid. Troubled by the attack, and her part in the defense of the sacristy, she turned to Mother Cailly and Father David for guidance.
In time, they helped her understand her gift and her new task. "Father David said it best," she thought. "If we're going to be one of the caretakers in the Garden, we shouldn't be afraid to roll up our sleeves and get our hands a little dirty." She put on her hood, her mask, and her gloves and went to do a little weeding in the Garden.