a plant which complements another plant in the landscape

A species used in a formal planting or border to enhance contrasts of height, color and leaf shapes.

Is a small plant displayed next to a bonsai. Accent plants are typically used when a bonsai is being formally displayed at a show or exhibition. Accent Plants can include any perennial, bamboo or grass.

A separate planting, often seasonal, of small herbaceous plants, bulbs or grasses, displayed in conjunction with bonsai when on formal display.

This could be a focal point plant. A plant to catch attention. Could also be called an anchor plant.