ive googled to see what is good with this plants and it just comes up with the same plants .I cant seem to find a good herb to go with your plant.
Try Lily of the valley , i know this spreads but it is a good companion for what you have got

If by companion herb you mean one that will keep pests and bugs at bay I don't think there is one. Companion planting tends to be done for vegetables more than flowers.