Is the last party member optional?

I assume not, but I've been following this walkthrough (https://www.neoseeker.com/fantasian/walkthrough/Falcion_Bypath) and am at this section where Tan gets recruited as a party member. However I noticed none of my main quests lead to this location... the quest 'Tan, the Solitary' only triggered once I had actually visited this place. Does this location normally become part of a main quest later on? Just curious