A HiveQL developer is responsible for managing and manipulating big data using Hive, a data warehousing tool in Hadoop's ecosystem. They write HiveQL queries, similar to SQL, to process and analyze structured and semi-structured data stored in Hadoop Distributed File System (HDFS). Their duties also include creating database tables, loading data, managing Hive partitions and buckets, optimizing query performance, and troubleshooting issues. They may also design and implement data models and schemas, ensuring data quality and integrity. Furthermore, they collaborate with other data professionals like data scientists and analysts to provide insights from large data sets.