That is indeed a begonia, Begonia angularis, a begonia from Brazil, named for its angular, ribbed stems. Can get very tall and tends to lean on other plants for support. Blooms white on peduncles reminiscent of other Brazilian thick stem begonias; blooms have been sparse for me in zone 8a. Begonia 'Grey Feather' is a listed hybrid, but may be a species variant; it has red backed leaves and red stems.
