Nowadays, the skyrocketing last-mile freight transportation in urban areas is leading to very negative effects (e.g., pollution, noise, or traffic congestion), which could be minimized by using autonomous electric vehicles. In this sense, this article presents the first prototype of Ona, an autonomous last-mile delivery robot that, in contrast to existing platforms, has a medium-sized storage capacity with the capability of navigating in both street and pedestrian areas. Herein we describe the platform and position it with respect to other existing prototypes, providing its main software modules and the first validation experiments, carried out in the Barcelona Robot Lab (Universitat Politcnica de Catalunya); Esplugues de Llobregat (next to Barcelona); and Debrecen (Hungary), which are representative urban scenarios. In such validations, we focus our analysis on the key localization module, whose errors could cascade down the rest of the navigation pipeline (e.g., planning or control). Aside from robotic technical details, we also include the results of the technology acceptance by the public present in the Esplugues de Llobregat test, collected in situ through a survey.