Hey Cassie! Will and Jem's parabatai bond was very strong. We also see Will's parabatai rune bleed when Jem was changing into a silent brother. Will felt his pain for him and wanted nothing more than to take it away. It was mentioned that Jem wasn't supposed to live as long as he did and that he was supposed to die years before that. I know we see from Emma and Julian that love from a parabatai can make magic. Could Will and Jem's parabatai bond helped Jem live longer than he was predicted?