• Windmark: ...Complicated. Coal & Wind first met when they were 14, bonding over their shared interests & hobbies, & sort of peer-pressured themselves into dating, before either of them were emotionally mature enough to do so. They never gave themselves time to really know each other as friends, first; instead essentially speedrunning into typical romantic couple things. 

  As time wore on, however; it became more apparent that their individual personalities just... weren't compatible for each other; like frost & fire. Windmark found Coalfang to be gratingly obnoxious & unfunny, while Coalfang found Windmark to be incredibly stuck-up & hypocritical. They tried putting up with the other for much longer than they should've, becoming slightly codependent, as they both feared being alone. Their relationship tipped over the boiling point when Sunriver drowned trying to save them both during a storm; causing the couple to break up on-the-spot, both blaming each other for her death; when really, it wasn't anyone's fault. 

  Now, they have an incredibly spiteful rivalry, taking every opportunity to take a mick out of each other. They do miss hanging out with each other & bonding over their interests, though; & their rivalry has sort of spun into a weird begrudging respect. While they'll obviously never be as close as they once were, maybe platonic friendship isn't out of the question.

• Raintail: Like fire & water; Rain & Coal are polar opposites in the best of ways; which led to them being the best & closest friends they've ever had in each other's lives. They fill each other's blind spots, cover for each other's flaws, bring out the best in each other; & are also both highly protective & supportive of each other. For better & for worse, they trust each other with their lives; & would willingly die for the other if it came down to it. They've known each other since they were kids; & their friendship's been growing strong ever since. (They both thought they might've had crushes on each other at some point, but then Rain figured out he was aroace, loveless, & romance-repulsed, while Coal had realised he might be bisexual (also that he was a trans dude) & the attraction he felt towards others while experimenting with his identity in those early days didn't feel like what he felt with Rain, so they talked it out & both agreed that they preferred each other as besties, anyway.)
  
    Nowadays, Coal sees Raintail as his cute little brother, ...despite Rain only being younger than him by about one year, & Coal being shorter in height. He's very affectionate towards Rain, both physically & emotionally- standing on his tippy-toes to ruffle his head-fur, pulling him by the sleeve when he wants to show him something, leaning over his shoulder, hugging, etc. He rants & rambles to him about whatever topic he finds interesting; & lights up like a Christmas tree on the rare occasions that Rain decides to ramble back. But as they've gotten older & he's gotten more aware of Rain's self-deprecating behaviour... well, it's hard for Coal not to worry; even if he'd probably ought to be a bit less... intense about helping him & gentler with how he tries to approach the subject. Coalfang is very ride-or-die when it comes to Rain & often just does things on his behalf that he thinks would help, without actually asking Raintail if that's what he wants. ...Which leads to Raintail retreating further in his own shell out of guilt, because he's worried about making Coal worried & escalating the situation further.
  
    Meanwhile, Raintail greatly appreciates having Coalfang in his life; so much so that he puts him on a bit of a pedestal, seeing him as infallible & invincible. ...Which unintentionally places pressure on Coal as the 'strong one' of the two. He cares deeply for him, though shows it in more subtler ways; & often ways that mirror how Coal interacts with him. Hearing Coal ramble about his interests always made Rain feel better, so when he notices Coal being upset, Rain'll try to do the same for him; rambling about whatever Rain finds interesting, to distract & cheer him up. Other than that, he's usually just happy to follow his friend's lead. He couldn't bear bothering his dear friend with his own dumb ideas- it's a miracle he even has a friend in the first place, isn't it?
  
    They're both just trying to help each other, but between Coal's fiery compassion & Rain's endless guilt, neither of them realise they're not obligated to always do so, or that it's okay to accept support from the other person, instead of only giving it- or even just step back & say "no". Friendship is supposed to be a two-way street & these two are crashing Ferraris into each other's metaphorical houses.