Sometimes it's the little things. Tiny things, really, in comparison to everything else. And this particular thing is practically minuscule, Will realises, compared to how many decades, almost centuries, the two of them have known each other. Minuscule even compared to how many years he's been watching them interact now.
And yet, years in, years of disliking Tesla and arguing with Helen about why she would possibly put up with him despite his continued shenanigans... Years in, it's this tiny moment that finally makes things click for him. This short, peaceful moment in time of him finding them in the library, finding Helen at one of the tables, surrounded by books, and Tesla standing right behind her. It's seeing the tiny act of Tesla twirling a lock of her hair around his finger, and leaning in to whisper something into her ear while she smiles patiently and pretends to continue to read that suddenly makes them make sense.
Makes Tesla make sense for her.
Because Will has never, ever, seen anyone else dare get that close to her, metaphorically speaking. Because, he realises with sudden understanding, Tesla is the only one in her life that takes great pleasure in unravelling the awe inspiring persona Helen Magnus has become, and takes even greater pleasure in cherishing what lies underneath.
And she lets him.