A Lambda developer is responsible for designing, developing, and maintaining serverless applications using AWS Lambda service. This involves writing code that's triggered in response to events, such as changes to data in an Amazon bucket or updates in a DynamoDB table. They ensure that the applications are scalable, fault-tolerant, and efficient. They also handle deployment, troubleshooting, and updating of these applications. Moreover, they work with other AWS services like API Gateway, CloudFormation, and IAM for creating secure and efficient solutions. Their role also involves optimizing costs, monitoring performance, and managing the application lifecycle.