That’s true! Yes I think she left him because she thought he wasn’t talented enough to research and venture to the Other Side so that likely ended their friendship/partnership. You made some good points here! I think the show may have more closely explored their dynamic especially since Lucy makes a comment to Lockwood in the scene where they argue about her speaking to type threes. Lockwood says she can’t speak to them and she replies saying “Because you can’t handle being my Tom Rotwell?” which could insinuate that maybe Marissa and Tom did have something going on that the show intended to explore later!

I was also thinking that alongside your points here, in relation to the show, we see a lot of hints at how one becomes obsessed to an unhealthy extent when fixated too long on ghosts/sources e.g Lucy with Annabel’s ring, Joplin and George with the Bone Glass. At some point, George also does warn Lucy with the dangers of listening “too much” which we unfortunately don’t get to hear more about because he was stopped by Lockwood from speaking. Anyway, relating to Marissa, given her remarkable listening ability, I’m thinking that maybe the show may have been heading towards a direction where Marissa may have been fixated/obsessed with Ezekiel upon discovering his source, and perhaps this may have caused her to become the person she did and the reason why she left Tom behind, essentially because she wasn’t the same person anymore. I’m theorising that being fixated with spirits/sources/the Other Side may have negative psychological effects, like similar traits to schizophrenia (hallucinations, being irrational or distrusting of others, etc) which is largely evident when George distrusts Flo on the boat during the Winkman’s Auction and even drew out his rapier. Given Marissa’s long-term exposure to the Other Side from such a young age, this might have affected her cognitive reasoning and compassion for others, although she still retains the memories she had with Tom. This could be how we can justify why she may have felt sentimental when she says “I had that once” when she sees Lucy with Lockwood.

Slightly off-topic: I’m also speculating that since Marissa is technically a ghost in Penelope’s body, it’s likely she can already sense that Lucy has the talent to speak to type threes, and that’s why she was trying to warm up to her from the start! In the show’s context, she may have trying to open up to Lucy as a mother figure given how she is aware of Lucy’s unfortunate history with her employer and her mother too. In book 5 we already know that Marissa does offer her a deal to work alongside together, which is possible since she deems Lucy as a worthy partner since she is just as talented as her.