Looks to me like a perennial aster.

I've got it too. I think it's a variety of smartweed, but that isn't much more than a guess. It is definitely a weed, though.

Look like asters to me also. In which case they could be something you want to keep - wait for the flowers to decide.

I just pulled out a couple hundred from one bed, spreading by underground runners. Definitely not something I want to keep :)

I haven’t seen that kind of segmented stem in an aster, but it does remind me of polygonum. Haven’t seen one this aggressive before though.

Yes, perennial asters, at least some of them, spread by underground runners. Their late beauty, when not many other flowers can be seen anymore, pays well for the little effort to control them. Mine look great when covered with a sprinkle of snow in November.