Ribbons for Remembrance.
Crisis Core reveals the source of Aerith's trademark ribbon, which she wears in all of her known costumes. While promenading around Sector 5, Zack and Aerith stop by a shop to look around. As a commemoration for their first meeting, Zack offers to buy her a gift. Aerith picks out a particular pink ribbon, which he ties in her hair.
The hair ornament has become one of the more famous symbols that represents Aerith. Throughout the series, as well as in other games such as Kingdom Hearts, she is shown wearing the same ribbon in every costume up to date. They were all designed by Nomura Tetsuya, and the representation of the ribbon adds a quite sentimental quality to her feminine appearance.
Despite not seeing Zack for years, she always wore the ribbon up until her death. Just like the ribbon, Zack has a special place; he was, after all, her first love. Although their relationship had indeed deteriorated due to Zack's prolonged absence, Aerith still made an effort to try and get a hold of him, wherever he may be. As time passed, and the events of Final Fantasy VII came to unfold, Zack seemed to slowly fade out of the picture. However, unbeknownst to all the players pre-Crisis Core release, the hair piece was not just a placeholder for the white materia; the ribbon constantly served as a memento of Zack's feelings for Aerith that were there from the start.
In Final Fantasy VII: Ultimania Omega, the production team confirms that the ribbons seen the remaining eight members of the party in Advent Children are worn in memory of Aerith. Each character dons a thin, dusty rose ribbon on their left arm. Except for Vincent, who
thinks he's too cool wears it on his right because his gauntlet is on the left.