Tester should play a key role in reviewing and validating the Requirement Specification document if it is line with the needs of the client.
The same holds valid for Design phase as well since all these validations will help him to create test plan and test cases to the highest standards.
Defects, if any exists in any phase of SDLC need to be reported instantly to avoid it carrying further to the next phases else it will be a huge loss to the company in terms of revenue as well as reputation.
His role extends in carrying out the core activities of the testing ranging from execution of the Test Assets to identifying the bugs in the application and reporting it to the developer either through an email or get it logged in a particular bug tracking tool.
The job of a tester does not end merely by logging a bug in the bug tracking tool, he needs to make sure that it is resolved by the developer as per the requirements of the client. For this, he needs to effectively communicate with the developer to get it rectified timely.
Being a senior tester, your role also extends to review the Test Assets created by the Junior Testers so that no requirements are missed however minor or major.