improver

Short definition from Wiktionary

Noun

1. 1
   Something that, or someone who, improves something.
2. 2
   Specifically, a person who improves land or property, a landscaper.
3. 3
   One who improves his or her performance, one who shows improvement (of individuals or groups).
4. 4
   A substance added to cause improvement (especially to a foodstuff).
5. 5
   A pad worn by women to make the dress hang properly.
6. 6
   One who performs improv; an improviser.