bearer of bad news
Literally, someone who delivers bad news. The bearer often identifies himself or herself as such as an introductory warning that he or she has bad news to deliver. I hate to be the bearer of bad news, but I'm afraid your grandfather passed away late last night. We were celebrating our teacher's absence until Susie, the bearer of bad news, told us that the principal was coming to give us our exam.
standard-bearer
1. A military member who carries the flag of his or her unit. The soldier marched proudly, flag in hand, as the standard-bearer of his unit.
2. The widely-accepted leader of a cause, movement, or ideology. She rose above her contemporaries to become the standard-bearer of the women's rights movement.