$ sudo apt-get update
$ sudo apt-get install lightdm-kde-greeter 

lightdm-kde-greeter is installed in your system.

Make ensure the lightdm-kde-greeter package were installed using the commands given below,

$ sudo dpkg-query -l | grep lightdm-kde-greeter *

You will get with lightdm-kde-greeter package name, version, architecture and description in a table

lightdm-kde-greeter package basic information: