Main characters in a story get the spotlight because it's their story being told, but also because their world is populated with a lot of other characters who support their stories. They can be characters that help move the tale forward, characters who have compelling backstory that provides a key detail for the future, and characters who stand in the way of the protagonist. In the case of Rooster Teeth's RWBY, there are many supporting characters that bring the world of Remnant to life.
The best supporting characters are the ones the audience wants to know more about. They are the characters the audience can't help hoping will appear in just one more episode, get a good flashback, or will become a larger part of the story.
10 Velvet
Velvet piqued the interest of the audience when she first appeared with rabbit ears as a member of the Beacon student body. Later revealed as a member of Team CFVY, Velvet has one powerful semblance, which allows her to use her perfect memory of others abilities as her own.
Velvet is a sweet and quiet teen, and it's almost strange to think she's trained to be a Huntress. She doesn't seem to enjoy it like so many other characters, and it only makes the audience more interested in her backstory and what she's been up to since the fall of Beacon in the third volume.
9 Sun
Sun starts his time on RWBY as a guy with a crush on Blake who doesn't seem to get that she's not interested in more than friendship. Once he gets over that hurdle, he becomes a breath of fresh and humorous air in an otherwise serious story arc.
Sun is the one character who is there for Blake when she leaves Beacon behind out of guilt for her potential role in its fall - but also in Yang losing an arm in a fight with Adam. Despite how heavy Blake's story can be, Sun lightens it, and also acts as a really great partner for some fantastic fight scenes. As the leader of Team SSSN, he, like Velvet, leaves Beacon after the fall, and fans would love the chance to see more of him in the show, not just the tie-in novels.
8 Mercury
What little the audience has seen of Mercury's backstory shows that he's had a difficult and dark upbringing. His father was a criminal who treated him horribly, but Mercury got his revenge. Instead of moving on from a life of crime and darkness, he falls in with Cinder.
Mercury and Emerald make for a great team when they're tasked with being Cinder's backup at Beacon Academy. Beyond that, they haven't had as much to do thanks to the larger focus being on Cinder and Salem. It's time for Mercury to get his chance at the spotlight as well.
7 Ilia
Ilia hasn't had as much screen time as any of the other supporting characters so far, but she's certainly a fascinating one. Like Sun, she plays a role in Blake's time at home. Ilia is Adam's new, young recruit, brought in to take Blake's place by his side.
She's impressionable, but a stellar fighter, and eventually, she understands that Blake is trying to do the right thing and Adam is just using her. Ilia learns a lot in her handful of confrontations with Blake, and it would certainly be interesting to see how her world has changed. It doesn't hurt that her chameleon abilities are ones the audience hasn't seen anyone else possess either.
6 Ironwood
Is he a villain, an ally, or just a man in a tough position trying to do what he thinks is right? Perhaps General Ironwood is all three. He begins the series as a military official in on Ozpin's plan, but eventually he becomes less of an ally for the titular Team RWBY and more of an antagonist.
Ironwood genuinely believes he's doing the right thing - even if the people around him don't. He's also a cyborg, and the audience has no background for just how he ended up that way.
5 Neopolitan
Neopolitan might be one of the most fun villains in the series. She seems to enjoy her work, whether she knows the whole plan or not. Previously working with Roman Torchwick, but by the seventh volume, working with Cinder Fall, Neopolitan makes for a great teammate for Cinder. She manages to question her judgment without ever saying a word at all.
She's also got one of the most powerful semblances in the series - able to change her appearance and everything around her to whatever she wishes. It makes her the kind of villain who can get in and out of areas completely undetected, and one that could do a lot of damage to the heroes.
4 Winter
Big sister to Weiss Schnee, Winter didn't get a ton of screen time until the seventh volume of the series. In an awkward position, Winter firmly believes in Ironwood, but she doesn't want to see her sister, or the synthetic being Penny, end up hurt as a result of Ironwood's plans either.
Winter grows up in the same house as Weiss, and probably has a lot of the same horror stories in her childhood. Fans don't get to see the Schnee family from her point of view, but maybe they should.
3 Raven
Much of Yang's childhood is informed by the fact that her mom leaves. Raven, Yang believes, does that a lot. She doesn't just abandon her daughter, after all, but her closest friends as well in the team that she trained with.
Raven, however, has a more interesting backstory than simply a woman on the run. She's the leader of a community of bandits, and she's one of the Four Maidens. Raven will likely become a much bigger player in the story as it continues, but for now, fans just want to know more.
2 Penny
A synthetic being built to be a protector, Penny has proven herself a "real girl" time and again. It's clear from the moment she and Ruby Rose become friends that Penny is going to be an interesting addition to the series. That's why fans were so heartbroken when Pyrrha destroyed her during the Vytal Festival.
Penny taking on the powers of one of the Four Maidens means she'll become an even larger part of the show. Her innocence and her being used as a weapon by the military makes for a strange juxtaposition, so her having some semblance of freedom in the eighth volume will give her a fascinating turn.
1 Qrow
Ruby and Yang's uncle has a bit of a checkered past. Like his sister, he comes from a community of bandits. Unlike his sister, he found his purpose in being a Hunter. Qrow also has the worst luck. That is his semblance, after all. That's why he tries to numb his pain and his awareness with alcohol.
Qrow has had a compelling journey as the series has progressed. Becoming a mentor to the teenagers at the center of the story, giving up drinking, turning his life around to make sure he does the right thing - right up until life deals him another blow at the end of the seventh volume. He loses an ally when Ironwood decides he and the teenagers are the enemy. He loses his newest confidante in Clover when Tyrian seizes the opportunity to kill him. Qrow is likely in for another dark arc.