Provided are a vinyl ether polymer which can exert temperature responsiveness even in the case of being supported on a highly hydrophobic fibrous substrate, an agent for controlling hydrophilicity and hydrophobicity containing the polymer, a method for controlling hydrophilicity and the hydrophobicity using the polymer, and a polymer-substrate composite having the polymer supported on a substrate. A star-shaped vinyl ether polymer which includes a central core and an arm portion bonded to the central core and in which the arm portion includes a repeating unit derived from a vinyl ether monomer and a structure represented by the following Formula (1) is included in the molecule: €ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ-Y-Ar 1 -N=N-Ar 2 €ƒ€ƒ€ƒ€ƒ€ƒ(1) i in which Y represents an ether bond, a thioether bond, a group -NH-, or a group -OR 1 -* (R 1 represents an alkylene group, * represents a position bonded to Ar 1 in Formula (1)), Ar 1 represents a substituted or unsubstituted divalent aromatic hydrocarbon group, and Ar 2 represents a substituted or unsubstituted monovalent aromatic hydrocarbon group].