Grass Rings.

They were kids, just kids, when Pieck's Mother told them they should marry thier best friend, and seeing eachother as exactly that, that is what they did, at the mere ages of 8 and 6, taking everything a little to literally. Porco fashioned rings of grass, and Pieck kept it, after all this time, even when she gets hurt in battle, Porco is there, physically or through his ring. The day is ingrained in both their memories, weather they like it or not.