  We address the question of description of qubit system in a formalism based
on the nilpotent commuting variables. In this formalism qubits exhibit
properties of composite objects being subject of the Pauli exclusion principle,
but otherwise behaving boson-like. They are not fundamental particles. In such
an approach the classical limit yields the nilpotent mechanics.
  Using the space of $\eta$-wavefunctions, generalized Schr\"{o}dinger equation
etc. we study properties of pure qubit systems and also properties of some
composed, hybrid models: fermion-qubit, boson-qubit. The fermion-qubit system
can be truly supersymmetric, with both SUSY partners having identical spectra.
It is new and very interesting that SUSY transformations relate here only
nilpotent object. The $\eta$-eigenfunctions for the qubit-qubit system give the
set of Bloch vectors as a natural basis.
  Then the $\eta$-formalism is applied to the description of the pure state
entanglement. Nilpotent commuting variables were firstly used in this context
in [A. Mandilara, et. al., Phys. Rev. A \textbf{74}, 022331 (2006)], we
generalize and extend approach presented there. Our main tool for study the
entanglement or separability of states are Wronskians of $\eta$-functions. The
known invariants and entanglement monotones for systems of $n=2,3,4$ qubits are
expressed in terms of the Wronskians. This approach gives criteria for
separability of states and insight into the flavor of entanglement of the
system and simplifies description.
