10. If they have one or more hatchlings, is it significant to them which of them bears the first egg?
There will be no eggs or hatchlings. There's simply no time for it. And by 'no time' I mean, the Driftwood-verse is set in the game's canon timeline.
IOW... they're only together for a few weeks before the supernova hits. And in this timeline, there is no 'everybody lives' ending...
Intimacy:
6. Who plays little harmless pranks?
Mostly Hal. They're still young and excitable and (being besties with Reg), are fairly sassy and cheeky by nature. Nothing big though (not like Reg's 'bucket over the ajar door' prank).
They'll usually do little things to get Slate's attention.
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11. How much time do they like to spend together? All, barely anything, or something in between?
They're both fairly busy people, so it's usually the just an odd couple of evenings a week. Not as much as they'd like, but a tree keeper's gotta do who what a tree keeper's gotta do...
Communication:
3. Do they disagree often? About big or small things? If they generally agree on things, is there at least one thing they disagree on?
Not really. Again, they're not together long enough for these sorts of things crop up.
But there was that one time Slate learned just how bad Hal is with technology and laughed at them a little. Hal probably got slightly miffed at that (not for long, though... Slate was standing behind their chair and leaning over them when it happened – very distracting :3)
Preferences:
3. Have they ever read each other's favourite book?
Slate doesn't read much outside of what's necessary to know how to do their job. They don't have the time for it either.
Hal spends most of their time studying linguistics and dendrology. Not sure if they'd dedicate any time to learning physics/engineering/advanced mathematics 🤔
8. Are they each other's "type"?
For Hal, absolutely. Genetic hardwiring again: Slate=tall, therefore, good match 👍 But Hal's pretty easygoing when it comes to personalities, so they simply like Slate because they're Slate.
Now, Slate on the other hand...
Hmm, I'd say that according to historical records, they have a thing for the cheeky outgoings types, so I guess Hal falls in that category 🙌
At least these two got to have fun for a little while 🥹