Chainfire managed to root easily and quickly the Nexus 9 and Nexus 6 without having any trouble. So the root is available, and the next step on the long way of developing is to install a Custom Recovery. This problem is solved by another XDA Senior Member, the Team Win Lead Developer and a Recovery Guru Dees_Troy.
TWRP version V188.8.131.52 handles perfectly Android 5.0 Lollipop with all its features. The new TWRP version automatically will decrypt the encrypted partitions using the “default_password” or promt for a password if it has been changed. In his project Dees_Troy implemented the most recent AOSP code into his project and fixed some annoying bugs, like cache wiping use to take a long time and exFAT issues.
What is TWRP
Team Win Recovery Project or shortly TWRP is a Custom Recovery designed to bring facilities to all it users. TWRP 2X or simply TWRP 2 is built with ease of use and customization in mind. It has a fully touch user interface and simpler in the same time so no more Volume buttons or Power button.
How To Install TWRP Recovery To Nexus 9 & Nexus 6
Almost all devices can be updated quickly and easily with TWRP Managger app.
Install TWRP Manager from Play Store .
Once installed, open it and provide root permissions.
Hit advanced > Install Recovery.
Verify the correct device name on the screen and then press Install Recovery when the device is showing.
If you have any question feel free to ask in the comment section below or visit the official thread.