Agree with Floral_uk, it looks like one of the black nightshades, probably S. douglasii given your location. There are several very similar species, and they grow wild pretty much everywhere. I take great pains to ensure that they never go to seed in my gardens, and they still crop up from dormant seed or from bird droppings. When I lived in San Diego, the local Hmong community cultivated one of the black nightshades extensively. There is a pretty god write up of the various species & their properties in the link below. Personally, I would just treat it like the weed that it is & dispose of it. Really nice jade plant bush behind it, by the way. Here is a link that might be useful: Black nightshade

Ken, thanks for your comment! LA is just initials from my name, I live in Lisbon, Portugal. I brought it inside just to see if it can do well since I like how it looks. It was outside under a tree, so it was only in full sunlight for no more than a couple hours a day. It was in an old pot and nearly dead, had no leaves on it and somehow survived. So even though I don't know what kind of plant it is I'm thinking it's very sturdy and should hopefully be OK inside in a hallway that doesn't get too much light? I might be wrong. Do you know what kind of plant it is?

I concur, that's a croton. They are used for outdoor landscaping all over Hawaii. Very hearty and doesn't get a lot of pests outdoors. It LOVES bright light. Good luck to you. :)

Yes, I discovered this plant as a "houseplant" decades ago at a nursery tending toward more unusual plants in the trade, and found it impossible to attain again in the northern plant trade. It's funny, I find that the thorns add a certain charm to the plant, but as said above, there are thornless ones available (in both dwarf and non dwarf). Both are suited for bonsai btw, and they have a very nice horizontal manner of spreading, almost like a ground cover, and aromatic flowers are a plus. I am currently growing the thornless non-dwarf. They should be more available than they are in the interior plant trade as they do quite well inside.