How can I use my LG K8 as a monitor for my raspberry pi?
Mobile screen has not any video input port which can be accessed easily so you can not use smartphone screen as the monitor.

You will require HDMI to screen ribbon cable converter which is neither standard nor available.

Edit:  You can start a vnc server on Raspberry Pi and connect both on same network. VNC viewer app

Or Use mobile to ssh into odroid and for X server forwarding use X Server XSDL app