Small Island Developing States (SIDS) such as São Vicente Island in Cape Verde face compounded energy challenges due to their reliance on imported fossil fuels, grid isolation, and vulnerability to climate impacts. While renewable energy offers a viable path forward, the variability of solar and wind resources necessitates effective long-term storage solutions to ensure reliability. This study assesses green hydrogen as a seasonal energy storage option and benchmarks it against battery-only and no-storage alternatives. The study assesses how São Vicente could be supported in reaching a 100% renewable energy system by 2040, providing the first green hydrogen storage assessment for a West African SIDS. A multi-scenario modelling approach was applied using the COMANDO (Component-Oriented Modelling and Optimization for Nonlinear Design and Operation) energy systems modelling tool, exploring four distinct 2040 system configurations: (i) PV and wind without storage, (ii) PV and wind with battery storage, (iii) PV and wind with hydrogen storage, and (iv) PV and wind with a hybrid battery-hydrogen storage. The systems were evaluated based on key technical and economic performance indicators, including Levelized Cost of Electricity (LCOE), Loss of Power Supply Probability (LPSP), and Total Annualized System Cost (TASC). A cost-sensitivity analysis was also performed to understand how system costs respond to changes in battery and hydrogen-related cost developments. Additionally, a qualitative assessment was conducted on enabling conditions such as infrastructure readiness, regulatory environment, and hydrogen development policies in Cape Verde, providing a broader view of the benefits of a circular economy as well as highlighting implementation barriers. Results show that the hybrid storage system (Scenario-iv) meets the technical criteria of 0% LPSP and 100% renewable share while maintaining a competitive LCOE of 0.0503€/kWh and a TASC of 6.97 million €/year. Compared to the battery-only scenario, the hybrid configuration reduces system costs by 25% and virtually eliminates unmet demand. Hydrogen plays a critical role during extended low-resource periods, demonstrating its value as a seasonal storage solution for ensuring reliability and flexibility. This study contributes to the growing discourse on smart, circular, and climate-resilient energy systems in Africa. It offers practical guidance to planners, policymakers, and investors looking to unlock the industrial-strategy potential of hydrogen in the energy transition of islanded and vulnerable regions.