I never use Siri, but when I look what has been draining my battery it says it's Siri!
In iOS 9, Proactive services like Siri Suggestions (on the far left home screen page) will consume power. It has nothing to do with the actual Siri.
If you don't like Siri Suggestions, you can disable it in Settings > General > Spotlight Search.
You might also go into Settings > General > Siri and make sure "Hey Siri" is turned off, especially if you have an iPhone 6s or 6s Plus. Your iPhone "listens" for the words "Hey Siri" but only when connected to power (unless you have an iPhone 6s or 6s Plus, then it's all the time), but doing the two things above and then restarting your device should prevent more usage from accumulating under the "Siri" bucket.
None of these suggestions helped me. What caused this problem was that my phone was set to automatically connect to a WiFi network that required a separate webpage-based login. They're called "captive networks" like the ones in restaurants and coffee shops, where you have to first open your web browser, and then fill in a login form, or click an "Accept" or "Go Online" button before you can connect to the Internet.
What happens is that my phone connected to the network automatically, but since I didn't open a web browser and manually login, I had no Internet connection. Then, Siri detected that I was connected on WiFi and tried over and over again to get out to the Internet, sucking up all my battery. (Perhaps Apple should have Siri do a progressive increasing of Siri's connection interval if the Internet fails? Kind of like Gmail does.)
To solve the problem, I just went into
Settings->WiFi and selected the WiFi network that was causing my problem, and then clicked
Forget this Network to stop my phone from automatically connecting. (You could also just leave your WiFi turned off, but that's no fun.)
Next day, I had 90+% battery life when I was heading home at 6pm.
I don't have an answer, but I have a theory. I'm having a hard time believing "Siri" is really "Siri". I have a phone that is waiting for a number port and sitting on my desk doing nothing. After 24 hours of sitting off Wi-Fi and looking for a network, not using it for anything, with the screen off, it says 90% of the usage was "Siri". Hence, I think "Siri" is synonymous with "System".