1. A copolymer, comprising two or more types of monomer units,
wherein
a hydration energy density of the copolymer calculated based on the following Formula (1) is 158.992 to 209.200 kJ-mol^-nm"3,
a monomer unit with a highest hydration energy density of monomer unit i calculated based on the following Formula (2) is a monomer unit not containing a hydroxy group,
a volume fraction of a monomer unit with a highest hydration energy density of monomer unit i calculated based on the following Formula (3) is 35 to 90%,
a difference in hydration energy density calculated by the following Formula (4) is 71.128 to 418.400 kJ-mol^-nm"3,
Hydration energy density (kJ-moW-nm3) of copolymer =
Z • =1{(moIar fraction of monomer unit i) x (hydration energy of monomer unit i)}/
£N {(molar fraction of monomer unit i) x (volume of monomer unit i)} ■■-Formula (1),
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-moW-nnr3) of monomer unit i = (hydration energy of monomer unit i)/( volume of monomer unit i) ■ ■ -Formula (2),
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 x
volume of monomer unit with highest hydration energy density of monomer unit i/
£j=1 {(molar fraction of monomer unit i) x (volume of monomer unit i)} —Formula (3),

wherein N and i are the same as defined above, and
Difference in hydration energy density (kJ-mol"Lnm"3) =
(hydration energy density of monomer unit with highest hydration energy density of monomer unit) - (hydration energy density of monomer unit with lowest hydration energy density of monomer unit) ■ ■ ■ Formula (4).
2. The copolymer according to claim 1, wherein
the hydration energy density of the copolymer is 167.360 to 200.832 kJ-mol^-nm"3,
the volume fraction of the monomer unit with the highest hydration energy density of monomer unit i is 40 to 80%, and
the difference in hydration energy density is 71.128 to 313.800 kJ-mol^-nm"3.
3. The copolymer according to claim 1 or 2, wherein the two or more types of monomer units comprise a hydrophobic monomer unit and a hydrophilic monomer unit.
4. The copolymer according to claim 3, wherein
the hydrophobic monomer unit is a repeating unit in a homopolymer or a copolymer obtained by polymerizing monomers selected from the group consisting of vinyl carboxylate, methacrylate, acrylate, and a styrene derivative, and
the hydrophilic monomer unit is a repeating unit in a homopolymer or 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, N-vinyl lactam and N-acryloylmorpholine.
5. The copolymer according to claim 3 or 4, wherein

the hydrophobic monomer unit is a repeating unit in a homopolymer obtained by polymerizing vinyl carboxylate or a copolymer obtained by copolymerizing vinyl carboxylate, and
the hydrophilic monomer unit is a repeating unit in a homopolymer obtained by polymerizing N-vinyl lactam or a copolymer obtained by copolymerizing N-vinyl lactam.
6. A separation membrane, comprising the copolymer according to any one of claims 1 to 5.
7. A medical device, comprising the copolymer according to any one of claims 1 to 5.
8. A blood purifier, comprising the separation membrane according to claim 6.