Th'ancred Nunh comes from a world where some peoples roles are swapped - but not their personalities, and so canon has shifted. Him and Y'shtola swapped. The other swaps?

Alphinaud == Alisaie

Cid == Nero

Th'ancred == Y'shtola

Urianger == Moenbryda

Aymeric == Estinien

Lucia == Livia

Regula == Zenos

Hien == Yotsuyu

The swaps aren't the only change, however - after 2.55? Minfilia didn't end up vanishing in Flow. No, Y'shtola gave her life to it... and Minfilia ended up needing to take up the role of Warrior of Light. For Twitter, Th'ancred is aware of how other worlds go. But he is a friendly purring catte with depression. Also he's Bolormaas' sibling, even if he's not the Siren.

Th'ancred, of the S tribe, challenged their leader and lost. Still feeling cocky, he left them. He was still a kitten at this time - still very young. He left with his brother, and hoped to attract ladies. (This, of course, was a failure - and he was still taken in by Louisoix. This time, however, for his mix of sneaking and magic prowess.) Yes, Th'ancred was young-ish when this happened. You complete dumbass of a Miqo'te.
 
Before the Calamity, Urianger was left in Sharlayan for his knowledge deciphering prophecies was still needed by many there. And Louisoix needed somebody with that mind to help take care of Alisaie and Alphinaud, should he fail... and Urianger took that seriously, even as he felt so angry and useless. (He went to Eorzea with them.)
 
Meanwhile, Cid Garlond failed in fleeing the Empire. Instead, he was taken into further to the Empire, Gaius refusing to let him go. That strength that helped him try and flee made him worthy of life, but... Meanwhile, Nero Scaeva took the chance to flee. But not out of disloyalty, but instead to see if he could infiltrate Eorzea himself. (It was only later he was dis-owned by Garlemald. He still doesn't believe he was.)
 
There's a lot more to put here - why certain people took certain paths - but... it's a LOT.
 
Only differences from canon will be noted. The Warrior of Light... well, to make it easier, they did the Gridiana opening, just so don't need to deal with the swaps in those storylines.
 
However, the first big difference does come before that, in the caravan in. Urianger wasn't talkative, but he was there. One twin on each arm. It was adorable.
 
Anyway, when dispatched to Thanalan, Y'shtola came with the Warrior of Light. She told them a lot about the theory of Primal, and how they work. However, the Warrior of Light still had to face Ifrit alone. During the fight, strange lazers seemed to also strike at the Primal. This was the work of Cid tol Garlond - who didn't just take readings. Instead, he also wished to see how fragments of the power of Ultima would harm the Primal.
 
Inconclusive. Thankfully, the battle was too chaotic, and the Warrior of Light didn't find out about this man.
 
When joining a Grand Company and going to the rememberances, Urianger was still with the twins - and still quiet. Reciting a prophecy, and leaving with them. The two do split, though. Alisaie realises that even if she doesn't like organizations, she can learn of what happened around the Calamity better if she tries working with them. Meanwhile, Alphinaud is disgusted at the Grand Companies and longs for something more... neutral. Urianger has trouble deciding which twin to go with, but he eventually goes with Alphinaud. Alphinaud needs more people with him. But Urianger wishes he could help them both... his heart longs for it.
 
After the Sylph quests, Minfilia is a little surprised Alisaie is the one asking her how the matter of Sylph and Ascian is going. But she suspects this has to do with the Calamity. If she must play the part Louisoix did, then she shall. But Alphinaud insisted he knew a better way, and so he's off. (Alisaie doesn't mention that Urianger is with him - she is unsure of the relationship that Urianger shares with the other former Archon.)
 
The Ala Mhigo/Hakkue arc goes around the same. Including Th'ancred getting a necklace with a Dark Crystal... but, due to his use of his magic, he resists the control for a time. Though he can't say it's happening - his voice seems to go when he tries. So, during the Titan Arc, there Th'ancred is trying to assist the Warrior of Light while being reminded of his failures. That his brother had gotten lost in the Empire, likely dead. That he couldn't handle this all. He was a failure of a Nunh, and a complete idiot. And eventually, Lahabrea took over - Thancred vanishing sometime while the Warrior of Light dealed with U'odh Nunh. Th'ancred had gone there, but that's what got to him...
 
So, yes - Th'ancred was still possessed by Lahabrea. This is also around the same time the Warrior of Light met Aymeric, the Azure Dragoon, for that questline. Though Aymeric is friendly, he still has to go on the run. Still stole one of Nidhoggs' eyes. (Somehow, this Warrior of Light is omniclassing everything. Just so the story parts can be included.) This also means that the Warrior of Light met Y'mhitra, and learned Y'shtola was a certain sort of information seeker. She used magic in her stabbers - a Rune Knight.
 
Anyway. After the Waking Sands were attacked, the Warrior of Light took refuge at the church. It was soon that Alisaie burst in, and revealed that the amnesiac man staying there was Nero nan Scaeva. Nero nan Scaeva was a traitor - and he's just as sassy and angry as the canon Nero. Well, a little calmer. However, the Ishgard Arc is very amusing - with, you know. Alisaie and Nero bickering.
 
More is gonna be added eventually.

Born in Sharlayan, Moenbryda was cheerful and assertive even as a child. She grew up fascinated by knowledge that surrounded her and was best described by her instructors as always eager to learn. Sensing in him the same draw to the unknown, she sought to befriend Urianger Augurelt, a quiet boy who far preferred the company of books to that of other children. With much aplomb, Urianger ignored Moenbryda's nigh-constant pestering; however, the stubborn Sea Wolf would not be put off so easily, and could oft be found lurking beside the boy, reading complex treatises over his shoulder. The two eventually grew close, and it was not uncommon to see the two late at night in the Great Gubal Library hunched over a stack of tomes and a smuggled plate of cockatrice meatballs.

In the time when Sharlayan sent envoys to Garlemald, her parents were among those who went - skilled diplomats who Moenbryda did not see eye-to-eye with. However, they did not return with the group. In fact, quite a few did not return. It was after this that she first knew loss, and first grew to hate. It was something she hid, however, and felt not many needed to know. Why would they?

Once she had grown, Moenbryda joined Urianger in studying under Louisoix Leveilleur - after assisting Urianger in entering the Studium. After demonstrating a striking talent in aetheric research, she became an Archon. However, while she did participate in the Circle of Knowing, Louisoix bid her remain in Sharlayan—an order she followed with great difficulty. Having seen twenty and nine summers, she has come to Eorzea at last to lend her expertise to the Scions of the Seventh Dawn. In times before Louisoix left for Eorzea, as an Archon, she also spent a lot of time with his grandchildren - taking care of them and seeing them as her own family, after her own losses.

After her sacrifice to stop Nabriales, Moenbryda shouldn't have woken again. But her aether - what made her, well, her was preserved. She thought it was a freak accident - and perhaps it was. Or perhaps it was a side effect of how she sacrificed herself. Either way, a gemstone-like crystal was left behind, and Urianger pocketed it. He didn't let anybody else know about it.

Still, he mourned. He thought that it was simply a remnant of her, a reminder. Time passed. He had begun to mess with new patterns for Carbuncle - if he could replicate his creation of a familiar out of Topaz for another gem, he could pass it on to Alphinaud. There were many sleepless nights as he played with gems, played with ideas. One day, he was playing with new ideas... and it reacted with the crystal which held Moenbrydas' aether.

She re-appeared, along with the Carbuncle made of her. Urianger didn't know how to explain, but he didn't need to straight away. Because, well. Moenbryda was just happy to be alive, at first. More... she thought she had been in a deep sleep. It took him time to tell her the truth. And now, well? She doesn't know what to do with herself.

ON HER GEM/CARBUNCLE: While Moenbryda is alive, yes, most of her aether is concentrated in the Carbuncle who follows her. If her Roegadyn body is destroyed, she can re-create it - and if her Carbuncle one is destroyed? It'll take some energy, but she can come back as well. If both of them are wrecked, she'll need somebody to summon her back up.

Her gem is currently being kept by Lyse, along with the spell documents.

• A Manikin is basically a crystal replica of a being, from Dissidia. Bolormaa is one made from similar matter to Hydaelyn.
• Technically, she comes from an alternate timeline. In her reality, time went back to 2.0 after 3.55 due to Urianger. Time only went back in the Source - but it needed to pull Minfilia in for it to exist as is had been. Thus, Bolormaa was made.
• In the fic she comes from, Animulae, she travelled and found out what she was. She also spent five years studying Time Magic to understand how the time travel happened, alongside Minfilia and Thancred. (Due to stronger forces, Thancred was also glitched into being a siren. She is aware of multiple versions of reality - well, she was told of the many Warriors of Light, and has simply picked up there's others like it.)
• ...Yes, she is only technically 5-7 years old.
• Bolormaa does not see herself as Minfilia, but also is unsure as to WHAT she sees herself as. However, she knows there is no sense lingering on that, and instead insists on aiding.
• Bolormaa IS NOT a Warrior of Light. In Twitterverse, she likely is working as a record keeper for the Scions. She is also aware that everybody, well, likely doesn't see her as she sees herself. She won't be hurt if people don't know her. She'll accept it and befriend them again, if she can. She also believes in the good of anybody. (In the verse where she is a WoL? This ended with her nearly dying to Zenos, because she refused to fight.)
• Her fic ended with her reality dying - you know, good sequel stuff - and her discovering her place was to be the same as Materia from Dissidia NT. However, on Twitter? Right now, Bolormaa is ignoring that's her place, and her responsibilities. To them, she is a soft sweet lady and not a newborn Goddess. I may canon update her later, once the fic begins.
• TECHNICALLY, SHE DOES NOT KNOW ANY POST-HW CONTENT FIRST-HAND. In her universe, the world ended before 4.0. This is actually a plot point for her fic. (Her fic is a meta fic.) However, due to RP elsewhere, she is a little familiar with the stories other Warriors of Light have told her. (This RP elsewhere is also why she's married to G'raha Tia.)
• Though she cannot fight for herself, she has a Carbuncle crafted from her shed scales.
• While Bolormaa looks like a Xaela, she took the Sagahl name because she feels it represents her. Also, she moves a lot like a Miqo'te.