The phrase "sweet fellow" is an informal expression used to describe someone who is kind, friendly and pleasant. It can also imply that the person is gentle or mild-mannered in nature.
Chester is a wonderful and sweet fellow who was found as a stray in Arkansas, and brought to a local shelter. (blinddogrescue.org)
The party's vice-presidential nominee that year was one David H. Koch, the younger of the now - famous Koch brothers, whom Haugh found to be «a sweet fellow.» (washingtonpost.com)