There are many KPI (Key Performance Indicators) suggested to measure the performance of software tester. My concern is what are the most useful/ efficiency KPIs for a tester?
There is never a general industry-wide answer to this sort of question. What might be "Key" for your company may very well not be "Key" for my company.
To answer this question for your specific context, find out what matters to the stakeholders you serve. Figure out what is important, what is not, and then determine what indicators can help you determine if you are on track or not.
And tread very, very carefully here. Depending very much on magic "Indicators" is very risky. Without a lot of context, such indicators can be very misleading. As just one example, if you determine that "number of bug reports written" is a key indicator, you may be emphasizing quantity and missing the quality of those bug reports. http://www.allthingsquality.com/2010/04/misuse-and-abuse-of-bug-counts.html
Also, remember that measurement systems always have side effects which can often lead to dysfunction. If you are in charge of implementing a measurement system, you owe it to yourself and your organization to read "Measuring and Managing Performance in Organizations" by Robert Austin first.
Agile doesn't support KPI applied on individual Software tester. Rather, you can apply KPI to measure whole team performance which include PM, Dev, testers etc. There are different factors like how many user-stories team handled in 2/3/4 week sprint, issues during sprint vs issues reported by customer etc. RCA(root cause analysis) is very important for each issue reported by customer so that we can improve the things in next sprint.