Ruth, Fiona & Helena. They work stunningly well with Greta & Mary Violet as the three names are strong stable classics. Out of those three I would be inclined to use Fiona because she's just as short and snappy as Greta but has the character and vitality of Mary Violet. Helena feels a little to frilly and lacks the snappy attitude that Greta and Mary Violet have. Whereas Ruth despite being gorgeous paired with Mary the biblical association maybe too strong.
Oh thanks so much for all the suggestions and name love! Greta, my oldest has her heart set on naming the little on Fiona should she be a girl which melts both my husband and my hearts. My husband has been pulling for Ruth and Lucy and keeps trying to convince her of them. She sweetly told him she had a great idea and that we should call her Fiona Ruth... smart little three year old.
I don't know what the heck it is with girl names but we have the hardest time picking. Each one of our girls had a solid name should they have been boys... Our poor Mary Violet had three other names before we felt right about her name.