"Si sente bene."
"One feels well" would be generic, I think, we would translate it as "Qualcuno si sente bene" (Someone feels well)
In Italian the personal pronoun can be implied, if we use no subject you imply then
io, tu, lui, lei, noi, voi, loro.
Another possible translation in English would be "He feels well", since the Italian sentence doesn't give any hint about the gender of the subject.