A copolymer having excellent water resistance and sebum resistance while also exhibiting high washability is provided. Also provided is a composition and cosmetic material containing the copolymer. The copolymer is generally polymerized from a monomer composition comprising: (A) a carbosiloxane dendrimer monomer having a radically polymerizable organic group； and (B) an unsaturated monomer having at least one acidic group or a salt thereof per molecule； wherein monomer (A) is present in the monomer composition in an amount greater than or equal to 30 wt. % relative to the weight of the monomer composition, and a weight ratio (A/B) of monomer (A) to monomer (B) is from 1.0 to 20.0.
