But it has serrated leaves and the plants I'm positive are echies don't. Its habit and dark stems also are not reminding me of them.

ruellia brittoniana?
Eupatorium serotinum?

Well, the leaves are too big to be that eupatorium, and they're not lance-y enough to be ruellia. Also, I've never seen ruellia growing here. I tried sowing some seed in a flat last year but never got sprouts.

Look at Rudbeckia.

Rudbeckia is similar, but I haven't seen any with such dark stems... Is it maybe a penstemon? I have some nearby, but I don't remember planting one there.

I was basing my guess for Rudbeckia on the vein pattern in the leaves. The one Rudbeckia with darker stems I can think of is R. triloba but my eyes aren't good enough to see any tri-lobed leaves in your photos, so I made a generic Rudbeckia guess.

Aha! I believe we have it--I did plant some R. triloba just couldn't remember where I planted. The habit is just the same and so are those dark stems. Thank you!

Oh, and it's not your eyes--mine doesn't have trilobed leaves, but from other pictures I can see that this characteristic varies.