Lamium first came to mind but I think it is dead mans nettle, non stinging variety, a pleasant insect friendly weed if I'm right. We have it & easy to keep in it's place if you like it. I do :-)

Dead man's nettle is right - which is Lamium. Pops up wherever it wants, spreads prolifically.

Never heard it called by that common name. I know it as Red Deadnettle (Lamium purpureum) and it's a true weed of cultivation with seeds that can lie for years in the soil until brought to the surface. Along with Petty Spurge (Euphorbia peplus) I think they are quite attractive though not in my garden!

I have white ones confined, along with mint, in a giant cartwheel laid in my paved courtyard....looks very pretty and keeps both under control

it's a weed :)