Rigorous coupled-wave analysis (RCWA) is the natural method for metasurfaces. It solves Maxwell's equations semi-analytically on exactly the periodic, layered geometry a metasurface has, and returns diffraction efficiencies, phase, and near-fields directly, fast enough to sit inside an optimization loop. Ikarus is built to make that power easy to reach, and to keep it honest. A forward simulation is a handful of readable lines of Python. A full inverse design is a handful more, with the solver choosing the right optimization engine on its own. Underneath the simple interface it is faithful by default: correct Fourier factorization, and convergence judged on the coefficients rather than the energy balance. The result that comes easily is also one you can trust. This paper walks through both halves. First, why that faithfulness is harder than it looks, and how we check it against independent solvers. Then how the same tool carries a design from a first forward sweep to a fabrication-ready, inverse-designed layout.