No login manager, sddm

Using KDE, you can go for 'sddm':
$ sudo apt-get instal sddm

Then, you need to check several things:
1/ Check you're booting into graphical mode:
$ systemctl status default.target
It should be 'graphical.target'. If not, go for 'systemctl set-default'

2/ Check the display manager is pointing to sddm:
$ sudo ln -s /etc/systemd/system/display-manager.service /usr/lib/systemd/system/sddm.service

Reference
https://bbs.archlinux.org/viewtopic.php?id=213460
https://wiki.archlinux.org/index.php/Display_manager#Loading_the_display_manager