Finally found it. Good heavensward.

So -


GETTING YOUR YO-KAI MINION

After completing the needed collection (13 minions for Whisper-go, 13 weapons for Whisper-a-go-go, 17 weapons for Jibanyan Couch)

1) Go to Old Gridania
2) Go to Apkallu Falls
3) Right click on Jonathas
4) Select 'Accept Achievement Reward'
5) Within submenu, select 'QUESTS' (because that's apparently what we consider things bought from coins earned in Fates, now - 'quests')
6) Your summoning item should be listed there.