So I haven't done Ember's questline yet so I don't know how that plays out but Isobel seems odd, despite liking her, it's weird that she'd join on as a companion just after she joined an Order of Knights (although it never specifies which one.. kinda have to blame the writers on not really giving her quest a good conclusion). But on Amenos, you encounter Meryline Barthel for a quest, and it almost seems like she's set up for a companion because at the end of her quest, she wants to be an adventurer and travel. It's like, set up perfectly for her to join you. You work well as a team, and she wants to travel. She'd be an interesting companion because of her unique singing magic ability... probably making her best as a healer, and I could think her unique ability would be decreasing monster aggro radius because of her song etc.
So was she thought of as a possible companion at one point and abandoned?