Roman Efremov contributed to the Kotlin/KEEP and google/kotlin repositories by designing and documenting language features and improving compiler reliability. He revived and expanded the explicit backing fields proposal for Kotlin properties, clarifying motivation and technical constraints to support maintainable code patterns. Roman also authored a formal design for named-only parameters, detailing migration and interoperability to guide future adoption. In google/kotlin, he fixed a false-positive in FIR analysis on the JVM, ensuring accurate identity checks for Java value-based classes. His work combined language design, static analysis, and technical writing, demonstrating depth in Kotlin, Java, and compiler architecture over two months.