East Ogan Komering Ulu (OKU Timur) in South Sumatra preserves Ulu script as a local cultural heritage. The increasingly limited use of the script motivates the development of digital media that can support recognition and writing on mobile devices. This study aims to design and develop an internal mobile virtual keyboard that accepts Latin-letter input, converts it into the OKU Timur Ulu script, and renders the output using a TrueType font. Research and Development (R&D) is used as the research method, while application development follows Rapid Application Development (RAD), consisting of requirements planning, user design, construction, and cutover/implementation. Flutter and Dart are used for the interface, a JavaScript module handles conversion, and a TrueType font provides internally mapped glyphs. Character data are adapted from documentation and prior research on OKU Timur Ulu script. Black Box testing of the seven documented scenarios shows that all tested functions produced the expected outputs. The study is limited to letter conversion; numbers and punctuation are not validation objects. Latin letters Q, U, Z, and X have no direct mapping in the implemented scheme and are handled through documented technical alternatives. The resulting Android application provides Latin-to-Ulu transliteration and an internal Ulu-script keyboard, with conclusions limited to software development and functional evaluation.