It was situated at a shady spot not far from the road.
At first sight I thought it looked a bit Begonia-like, a bit succulent, but at a closer look the leaves are arranged opposite, so that would rule that genus out.
No flowers to help us I'm afraid..

It looks a lot like a Gloxinia perennis plant that I used to have. It is native to that area.

I think you're spot on Danita!
This photo on the web convinced me looking at the leaf shape, stem and the little plantlets in the axels..
http://cookislands.bishopmuseu...

It is native in Brazil, not in the Rio de Janeiro state though where I found it, but in the close neighbour state; Minas Gerais.
And being along the road can explain how it got there..

Ooo I love that thing with it's big purple bell flowers, unlike most Gloxinias they do very well in tropical garden, surviving pouring rain. Easily propagated with rhizome too.

I'm glad I took a little piece of it home, it is doing well so far, so I can look out for its beautiful flowers later on!