The additional bitter herb on the Seder plate, often romaine lettuce, is Chazeret.
Chazeret is one of the traditional symbolic foods associated with the Passover Seder. It is generally represented by romaine lettuce, whose roots become bitter, though endive and other bitter greens may also be used. The item recalls the bitterness and harshness of Israelite slavery in Egypt.
Chazeret is commonly distinguished from Maror, another bitter herb on the plate. In many traditions, Maror is represented by horseradish, while Chazeret is used for the korech, or Hillel sandwich, made with matzah and bitter herbs. Practice varies among Ashkenazi, Sephardic, Mizrahi, Persian, and other Jewish communities, so the exact plant and arrangement may differ.
Karpas is a separate vegetable dipped near the beginning of the Seder, usually in salt water or vinegar, and Charoset is the sweet mixture associated with mortar and brick. Those symbolic foods are therefore not substitutes for the specific Seder-plate term asked for here.