You need to advance the events of Ryoko and Elly to get to a hidden base in the south and meet the researcher Cassedy. The events with Cassedy will introduce Scuffy.
What you have to do depends on how far you're with there events, but there is a step by step guide in the wiki:
Ryoko: https://renryuu.miraheze.org/wiki/Ryoko#Character_events_sequence
Elly: https://renryuu.miraheze.org/wiki/Elly#Character_events_sequence