A copolymer, comprising two or more types of monomer units, wherein the two or more types of monomer units comprise a hydrophobic monomer unit and a hydrophilic monomer unit, the hydrophobic monomer unit is a monomer unit with a lower hydration energy density than that of a hydrophilic monomer unit and is a repeating unit in a copolymer obtained by polymerizing monomers selected from the group consisting of vinyl carboxylate, acrylate, and a styrene derivative, the hydrophilic monomer unit is a monomer unit with a higher hydration energy density than that of a hydrophobic monomer unit and is a repeating unit in a copolymer obtained by polymerizing monomers selected from the group consisting of an allylamine derivative, a vinylamine derivative, N-vinylamide, an acrylamide derivative, a methacrylamide derivative, and N-acryloylmorpholine, and wherein a hydration energy density of the copolymer calculated based on the following Formula is 167.360 to 209.200 kJ·mol -1 ·nm -3, the monomer unit with the highest hydration energy density of monomer unit i calculated based on the following Formula is a monomer unit not containing a hydroxy group, a volume fraction of the monomer unit with the highest hydration energy density of monomer unit i calculated based on the following Formula is 35 to 90%, a difference in hydration energy density calculated by the following Formula is 71.128 to 418.400 kJ·mol -1 ·nm -3, Hydration energy density kJ ⋅ mol − 1 ⋅ nm − 3 of copolymer = ∑ i = 1 N molar fraction of monomer unit i × hydration energy of monomer unit i / ∑ i = 1 N molar fraction of monomer unit i × volume of monomer unit i wherein the hydration energy of monomer unit i is an absolute value of a value obtained by subtracting energy in vacuum of monomer unit i from energy in water of monomer unit i, N represents a total number of monomer species constituting the copolymer, and i represents an integer of 1 or more and N or less, Hydration energy density kJ ⋅ mol − 1 ⋅ nm − 3 of monomer unit i = hydration energy of monomer unit i / volume of monomer unit i Volume fraction % of monomer unit with highest hydration energy density of monomer unit i = molar fraction of monomer unit with highest hydration energy density of monomer unit i × volume of monomer unit with highest hydration energy density of monomer unit i/ ∑ i = 1 N molar fraction of monomer unit i × volume of monomer unit i wherein N and i are the same as defined above, and Difference in hydration energy density kJ ⋅ mol − 1 ⋅ nm − 3 = hydration energy density of monomer unit with highest hydration energy density of monomer unit i − hydration energy density of monomer unit with lowest hydration energy density of monomer unit i the two or more types of monomer units comprise a hydrophobic monomer unit and a hydrophilic monomer unit, the hydrophobic monomer unit is a monomer unit with a lower hydration energy density than that of a hydrophilic monomer unit and is a repeating unit in a copolymer obtained by polymerizing monomers selected from the group consisting of vinyl carboxylate, acrylate, and a styrene derivative, the hydrophilic monomer unit is a monomer unit with a higher hydration energy density than that of a hydrophobic monomer unit and is a repeating unit in a copolymer obtained by polymerizing monomers selected from the group consisting of an allylamine derivative, a vinylamine derivative, N-vinylamide, an acrylamide derivative, a methacrylamide derivative, and N-acryloylmorpholine, and wherein a hydration energy density of the copolymer calculated based on the following Formula is 167.360 to 209.200 kJ·mol -1 ·nm -3, the monomer unit with the highest hydration energy density of monomer unit i calculated based on the following Formula is a monomer unit not containing a hydroxy group, a volume fraction of the monomer unit with the highest hydration energy density of monomer unit i calculated based on the following Formula is 35 to 90%, a difference in hydration energy density calculated by the following Formula is 71.128 to 418.400 kJ·mol -1 ·nm -3, Hydration energy density kJ ⋅ mol − 1 ⋅ nm − 3 of copolymer = ∑ i = 1 N molar fraction of monomer unit i × hydration energy of monomer unit i / ∑ i = 1 N molar fraction of monomer unit i × volume of monomer unit i wherein the hydration energy of monomer unit i is an absolute value of a value obtained by subtracting energy in vacuum of monomer unit i from energy in water of monomer unit i, N represents a total number of monomer species constituting the copolymer, and i represents an integer of 1 or more and N or less, Hydration energy density kJ ⋅ mol − 1 ⋅ nm − 3 of monomer unit i = hydration energy of monomer unit i / volume of monomer unit i Volume fraction % of monomer unit with highest hydration energy density of monomer unit i = molar fraction of monomer unit with highest hydration energy density of monomer unit i × volume of monomer unit with highest hydration energy density of monomer unit i/ ∑ i = 1 N molar fraction of monomer unit i × volume of monomer unit i wherein N and i are the same as defined above, and Difference in hydration energy density kJ ⋅ mol − 1 ⋅ nm − 3 = hydration energy density of monomer unit with highest hydration energy density of monomer unit i − hydration energy density of monomer unit with lowest hydration energy density of monomer unit i