Weed or flower
There is so much going on in these photos, it's hard to see individual plants, but we think we know which one you mean. Looks like lespedeza. There are several species, but it's a good chance, if it is a volunteer, that this is one of the non-native ones that has become invasive, e.g. Chinese lespedeza. Do not let it go to seed. Chinese lespedeza should be destroyed.
You are welcome to send us a photo of it in flower, but, again, do not let it go to seed.