How did Sanguine become friends (or at least acquaintances) with Renwick?
Sanguine and Lines happened to be in the area when Lines got sick with something flu-like and they sought out Renwick and Luz because they often provide medical care free/cheap.
They didn’t really have anywhere to go after they’d been seen, since they were homeless vagrants at the time and Sanguine convinced Renwick and Luz to let Lines stay in the clinic until he was in better shape, which they agreed to since they weren’t familiar with Saratoan biology and weren’t sure exactly how safe it was to send him back out on the streets with just some medicine to reduce the severity of the symptoms. Sanguine had to be allowed to stay there as well to be Line’s translator (Lines is deaf and mute).
Sanguine made friends with Renwick while he was staying there (actually he made friends with half the Dryads in the area. He’s that kind of gregarious smooth talker).
What prevented Renwick from moving his tree closer to his current residence in med school?
Well for either Renwick or Luz, they couldn’t find any property near enough to the school that would allow them to plant a tree temporarily, since that would involve digging a huge fucking hole and then putting a tree there that can not be allowed to become the property of the owner of the land. Basically no one within a close enough radius of Luz’s chosen school would allow it - there were some closer options, but far enough out of range to be somewhat pointless.
Also keep in mind that Dryads are insanely protective of their trees. There were some odd rare situations where Luz could have planted his tree somewhere but didn’t trust the property owner and was basically being insanely paranoid, and didn’t feel comfortable leaving his tree where neighboring Dryads couldn’t keep an eye on it while he was gone.
Where and how did Luz and Ren first meet? I'd guess med school since they're both doctors (of a sort). Also, what kind of doctor is Renwick?
Luz and Renwick come from the same general area and sort of knew OF each other since childhood, but they weren’t really acquainted. Their parent’s trees were far apart, their parents didn’t know each other, they kind of just didn’t have any reason to interact outside seeing each other occasionally when all the local Dryads gathered together and all the children were gathered together in a small crowd. So that’s when they met, but when they met for real, actually becoming friends and then later a couple, was when Luz went to medical school - but not AT medical school.
Renwick had an interest in making medicine for a long time and had started to apprentice under another Dryad to learn their folk remedies, but he’d also wished he could learn about human pharmacological medicine, but didn’t think he could do that. The injury his tree had sustained and its effect on him made him extremely cautious, which left him more acutely aware of being a bit stuck where he was.
While that was happening, Luz, separately, had decided he wanted to become a surgeon. He wanted to cut people open and save lives! (those were both important details of the dream. Surgery is kinda weird when you think about it) And he’d go to medical school and learn how to do it if it killed him! (and it nearly did) Suddenly nameless Dryad in the community number 321 became “That crazy willow kid who’s going to medical school”
Ren immediately sought Luz out when he heard about this, and would sit and wait by Luz’s tree every time Luz came home for a break from medical school so he could go on talking to him (and also read his textbooks). They hit it off famously.
Luz also paved the way for Renwick to go to medical school too. He grappled with the system to find a balance between needing to be there, needing to not keel over and needing them to not expel him because of the breaks. And he convinced Ren he could do it too, so by the time Renwick gave in and tried medical school too, they were better prepared for a Dryad with more delicate health who wasn’t as willing to flirt with death (if he’d ever been threatened with expulsion, he probably would have given up rather than go and talk to every possible person in the chain and attempt to charm them to death - which is what Luz did whenever he met a wall).
Renwick is an apothecary/pharmacist.