Summary: This paper is the first part of a trilogy dedicated to the following problem: given spherically symmetric characteristic initial data for the Einstein-Maxwell-scalar field system with a cosmological constant { {\(\Lambda\)}}, with the data on the outgoing initial null hypersurface given by a subextremal Reissner-Nordström black hole event horizon, study the future extendibility of the corresponding maximal globally hyperbolic development (MGHD) as a ‘suitably regular’ Lorentzian manifold. In this first part we establish well posedness of the Einstein equations for characteristic data satisfying the minimal regularity conditions leading to classical solutions. We also identify the appropriate notion of a maximal solution, from which the construction of the corresponding MGHD follows, and determine breakdown criteria. This is the unavoidable starting point of the analysis; our main results will depend on the detailed understanding of these fundamentals. In the second part of this series [the authors, “On the global uniqueness for the Einstein-Maxwell-scalar field system with a cosmological constant. II: Structure of the solutions and stability of the Cauchy horizon” (2014), arXiv:1406.7253] we study the stability of the radius function at the Cauchy horizon. In the third and final paper [the authors, “ On the global uniqueness for the Einstein-Maxwell-scalar field system with a cosmological constant. III: Mass inflation and extendibility of the solutions” (2014), arXiv:1406.7261] we show that, depending on the decay rate of the initial data, mass inflation may or may not occur; in fact, it is even possible to have (non-isometric) extensions of the spacetime across the Cauchy horizon as classical solutions of the Einstein equations.