Sisu is generally considered the worst dragon to be the last, be it in-universe or from audience reaction, showing a level of naïveté that overlaps with Too Dumb to Live. This actually foreshadows the fact that the real reason why the Dragon Gem worked is because of her family's trust in her, not because of Sisu herself, and anyone who fulfils that condition can activate it as well. This was what Raya realized in the climax.
Sisu is a water dragon. The Druun are repelled by water. Is this fact part of why her siblings chose her, trusting that the Druun's natural avoidance of her element might protect her long enough to activate the gem...or are the Druun repelled by water the second time around because Sisu is a water dragon? The most likely answer, of course, is: Yes.
Why did the dragons remain turned to stone after the initialWorld-Healing Wave in the prologue? Because, as demonstrated when Kumandra split into five tribes, human disharmony still existed. The Druun get their strength from humanity's collective distrust towards each other. If not for the Dragon Gem breaking, the dragons remaining stone is a testament that humans still needed to trust more. Once Raya and her friends trusted each other and Namaari, they were able to fully banish the Druun's affect on humans and the dragons.
Raya opening up to Tong when she learns he's alone and lost his family seems almost out of character for someone who spent six years of her life believing that trust is dangerous at best. But then it occurs why that endears Raya to him. Raya is the last of the people of Heart, and aside from the fact he's not the Leader of Spine, she can relate to Tong when she sees he's the last of Spine as well.
The five tribes being named for different parts of a dragon becomes very meaningful when one realizes each people reflects that part.
Tail may not seem like much, but a tail is very important to an animal because it allows it balance and (in Sisu's case) the ability to steer in the water. And just so, Boun runs a boat and knows how to steer it.
Spine is one of the sharpest parts of a dragon, fitting for a warrior clan that knows nothing except battle and combat. Loneliness aside, one gets the idea that Tong is the way he is because of so much fighting.
"Spine" is also commonly used as a metaphor for courage and determination. Tong was determined to defend the homes of his friends and neighbors even after they were all statues, and had the courage of a seasoned warrior.
Talon can be alluded to "sticky fingers", as its people are often looked down upon for being thieves and pick-pockets. Little Noi embodies this trait very well.
Fang is a two-fold meaning for Namaari's clan. Given her people's reputation for backstabbing, it alludes to the saying "biting the hand that feeds you". Alternatively, fangs can be found inside the head, something the pragmatic Virana and her people use to get what they're after.
Heart is rather self-explanatory. Raya's father, despite being capable of killing the other leaders who tried to steal the dragon gem, chose a peaceful option.
Notably, the trope Liar Revealed is zigzagged in the story. On one hand, you have Sisu who could've easily covered up that she was the least powerful of the dragons, but instead averted this trope and was upfront to Raya. On the other hand, you have Raya who played the trope straight and covered up Sisu's dragon nature until the truth revealed itself. Naturally, their friends are not happy with her when they learn she kept the dragon a secret from them. This Foil is meant to show the follies of not trusting your allies, especially with the truth.
If one notices carefully, Raya earns all her human allies by unknowingly giving them each a token of trust. She gave Boun half her money (and promised the other half), a transaction that couldn't have been done if she didn't want to trust him at all. Although trusting them was initially a mistake, Noi and the Ongis opened up to Raya when she promised them food in exchange for their services. Heck, Tong even received Raya's sympathy when she learned he was the last of Spine and lost his family to the Druun. And when Raya gave Namaari back her dragon pendant, it proved (almost) successful in gaining her trust. But at least trusting her with the Dragon Gem shards lead to her choosing tosave the world. Perhaps Sisu's "gifts-equal-trust" philosophy wasn't as crazy or naïve as Raya thought.
In the beginning of the movie, when Namaari betrays Raya, she kicks away little Tuk Tuk for trying to stop her. Later, towards the climax, she saves Tuk Tuk from being turned to stone by the Druun, redeeming her from her earlier actions six years ago. This is meant to lead up to how Namaari will redeem herself for betraying Raya (specifically by putting the Dragon Gem back together).
Sisu not knowing about Toot-and-Boom beetles has an air of innocence, but there's also a tinge of brilliance (and horror) as to why she wouldn't be familiar with this insect. If it's been 500 years since the initial Druun attack, there's a chance that during the war between the five tribes, one of them probably bred this animal for the purpose of warding off enemies. Understandably, Sisu wouldn't know about these little guys because she hails from a time period when Kumandra was a peaceful land.
When Raya is describing each of the other tribes to her father, there's a Description Cut from a Fang cat viciously hissing to little Tuk-Tuk hissing like-so. This may seem funny on the surface, but it could foreshadow what Raya comes to learn throughout the movie: the very tribes she once thought to be her enemy are not that different from her or people.
The relationship between humans and Tuk Tuk's species is truly symbiotic. Tuk Tuk is faster and better protected when he curls into a ball and rolls, but he can't see so he needs Raya to guide him. It's a relationship that requires trust, one of the movie's central themes.
Having Namaari be the one to actually reassemble the Dragon Gem and save the world instead of Raya not only affirming the Character Development of both characters (Raya learns to trust someone she once hated, and Namaari finally gets over the pragmatic ideology and do the right thing) but also indirectly fulfilling Virana's wish as well. Virana wants the Fang tribe to be the one to save the world to redeem themselves in the other clans' eyes. Namaari's What You Are in the Dark moment means a Fang did end up being the one to save the world after all, restoring their reputation with the other clans in the ending.
When Sisu in human form gets in trouble at the village for pretty much shoplifting items after receiving bad advice from Boun, Dang Hu shows up and easily pacifies the crowd while leading Sisu away and none of the shopkeepers raise any further voice in complaint. At first glance, one might not think too much of this or at the least, be misdirected into thinking Dang Hu is the grandmotherly village matriarch figure who has everybody's love and respect. However, after it's revealed Dang Hu is actually the ruthless Talon chieftain who leaves Sisu to die at the hands of Druun, the real reason all the villagers stood down wasn't for mundane reasons... but because they knew who Dang Hu was and they were terrified of her.
Namaari's Chronic Backstabbing Disorder is borne partly out of her upbringing in Fang, and partly as a consequence of being branded as the person who broke the world by everybody else. Aside from her own people in Fang, literally everybodyhates her for what she did, which could be a major reason why she seems to be on edge and prone to making rash decisions all the time. For six years, she believes that the entire world is out to get her and she could trust nobody but her own people. Her mother's cynical summation of how the world blamed Fang for breaking the world didn't help matters, which culminated in her not willing to buy Raya and Sisu's peace offering and brought a crossbow with her, with disastrous results. In a way, Namaari is not so different from Raya as they're both victims of their own mistrust of others, even if in Namaari's case it's mostly her own fault.
More on Namaari and Raya being not so different: Raya's A Minor Kidroduction training scene has her attempting to breach the defenses in the Dragon Gem temple and overcome its guardian, aka the exact same thing Namaari and potential gem hunters ended up doing.
Namaari and Sisu needed to be the ones to activate the gem for the exact same reason: Trust. Sisu was by her own admission the least of her kind; the youngest, the most naive, the least powerful. Namaari is "the girl who broke the world." They were both, in the moment the gem needed to be activated, the last logical choice to trust with anything important. And without trust, real trust, the gem will not work. Sisu was wrong. It couldn't have been "any of us." It had to be the one that required the biggest, most genuine leap of faith.
Sisu and Namaari are not comparable: Sisu's siblings knew her since birth, so of course they could trust her. Namaari commited betrayal twice, the first time ended the world, and hunted Raya down for years.
The five dragons worked together to create and activate the Dragon gem that initially banished the Druun. Later, it broke into five pieces, which conveniently enabled each of the five tribes to claim one piece and work together at the end of the story to banish the Druun a second time.
In the finale when everyone is reunited with their family, it's around the area where Raya found them. Boun in particular - he returns to the exact spot where Raya met him. Because his family was nearby there.
Trust will block out discord, and so the other dragons trusting Sisu was enough to activate the Gem and banish the Druun. But in order to "undo" the damage of discord, you need not just trust but forgiveness. Raya and the others were willing to forgive Namaari, to give her their gem shards even though she had betrayed them, and that is why the second activation of the Gem could restore those who have been petrified.
Considering how much time passes before Raya gets a hint of how to summon Sisu, and that Tuk Tuk has grown from a tiny armadillo creature to a full mount, she may have had to struggle with starvation from time to time.
Benja knows that the Druun can't cross water, given he tosses Raya into the river to save her life as a child, and that the Dragon Gem doesn't confer prosperity on the owner. It's implied he knew that a great show of trust would repel them once and for all, and he was going to explain this at the big feast. It was naïve effort, but a noble one. Namaari and Virana's actions doomed what would have been a peaceful way to stop their common enemy, and Namaari realizes it in the climax.
How sturdy is the stone of petrified humans? It's safe to assume that they wouldn't suffer from erosion of wind or water but what if a natural disaster happened? If a statue fell into water by accident would the person be drowning when revived? What if they were buried by a landslide? Or what if there was some force so hard that a statue could shatter?
