It has a very dark leaf and is quite a substantial sized plant. Can you remember what was in the pot last year?

any chance of a closer view of the leaves and tip if there are any buds forming?

and if you don't want the plant it is technically a weed :o)

Can't think offhand of a weed that looks like that. Wait until if flowers and as soon as it does put on another photo before it sets seed, just in case!

Michelmas daisy ???

Penstemon or lobelia?
Definitely looks as though the colour of the plant complements the pot, so I reckon it should stay until you decide you hate it or need the pot for something else.

Hi, I think my guess would be Lysimachia ciliata 'purpurea', Derek.