This is a concept I've talked about briefly but not with much detail.
I loved the first book, of course. I was prone to love it thanks to the scholarly bent of the prose, the focus on fairies, and Emily's absolute nerdery and awkwardness. But talking about how much you love a protagonist is for another day. What Heather Fawcett does in addition to having a pairing I actually like is add side characters who can fill out the world in addition to Emily and Wendell.
In the first book, this role was taken up by the townspeople, and it flowed alongside the episodic nature of Emily's research. She's simply trying to gather information about the types of fairies seen in a Scandinavian town. This means the bulk of her interactions, and hurdles, come from the various denizens of Ljosland before Wendell even shows up. Emily's brusque, professional personality puts the people of Ljosland off at first, leaving her without their help in an extremely unforgiving environment. There are plenty of saves in the novel, but the biggest one has to be when Wendell is able to help her get along with the townspeople better thanks to both his charm and inadvertently getting injured and causing her to beg for their help. Life gets so much easier afterward.
Notables here include Aud, the leader of the village that Emily offends on day one. Also Thora, who is an elder who doesn't mince words, which makes her the easiest person for Emily to get along with. Additionally we have patient and woodchopping Lilja, who Emily takes to help save her fiancé Margret from the faeries. A major shoutout has to go to Poe, though, a tree-dwelling brownie who Emily befriends early on and who proves to be incredibly helpful to this Noser asking him all these questions.
I have to love Poe. It's a testament to her worldbuilding that Fawcett can indicate both the ethereal beauty of the fairies as well as describe ones like the brownie as so ugly to our eyes they might be cute. And DANGEROUS. Even the tiny Poe has sharp, needle-like fingers. I cannot tell you my delight when he showed up again in the next book. I hope he continues doing so.
Since Emily is a researcher, our next book demands a different setting, first in Cambridge and then off to their next research location, which appears to be also cold, but not nearly as cozy as Ljosland since Emily and Wendell don't bond with the locals as well as they did before. This leaves us with a new cast to fill out, lest the interactions only develop between Wendell and Emily, and as entertaining as they are, it would get one-note before we can reach the resolution. Thus, we are introduced to Ariadne, Emily's niece, and the head of her department, Farris Rose.
Ariadne is a great foil for Emily, possessing the single-minded scholarly obsession but with a youth and cheerfulness that makes her almost an annoyance to Emily more than anything else. Since Emily is so bad at social interactions, Ariadne gets along better with Wendell, actually. Rose, on the other hand, provides another type of conflict: he's older and has more authority in the department. He's introduced wanting to fire both Emily and Wendell, but then worms his way into their expedition because he's as ambitious as she is. Unfortunately, his methods are a bit... dated.
These new characters provide Emily people to bounce off of who aren't just... there. They challenge her, frustrate her. Part of her arc in this book comes to learning to deal with the other scholar and her niece. This is on top of the problems she already has.
What we an learn from these uses of additional characters is thus:
1) Take your time. Rose and Ariadne are influential and important characters, and they don't even show up in the first book.
2) Allow secondary characters to connect to your characters' backgrounds. Ariadne's existence perpetually reminds us of Emily's social difficulties and enables us to think of her in a more complex way than a disembodied scholar or love interest.
3) Repeat when necessary. I was overjoyed for Poe to come back. He's not the only lesser fairy that Emily interacts with. She finds one of the creepy, carnivorous fox fairies to help her return to Ljosland briefly so she can meet with Poe. This fairy comes back around later on to help them.
4) Piggybacking off #3: These secondary characters have their own goals and motivations. With the fairies, it's helpful that Emily as a scholar understands their rules more so than with the people. The fox fairy (who she names Snowbell) follows them just because he's curious, but that doesn't mean that he cares about the quest itself. He's perfectly happy for Emily to be eaten if he can continue the quest with Ariadne.
But then you have the human characters and their motivations are clear enough, since both are scholars and would want the chance to observe what Emily is finding. Still, Rose manages to be more of a hinderance at times, and Emily becomes conflicted about Ariadne's presence because it is dangerous to seek fairy doors in the way they are.
Each character always has their own point of existing outside of the protagonists. But at the same time, they provides ways to stymie or push your character forward. It's complex, but the longer your series goes on, the more people there will make the world feel full and real. Try to consider both as you add secondary characters to your world. Who are they? What do they want? Need? Fear? Test them as much as you test your protagonists. We need them to hold up their end of the plot.