Thoma (托马), the protector from afar, the USA soldier stationed in Japan.
When comrade Thoma debuted in 2.2, he was heralded as the next best Hu Tao support, being better than zhongli as he allowed Hu Tao to swirl pyro more easily, a feature lacking in many Hu Tao-Morax teams. Thoma also provided pyro resonance, boosting Hu Tao's atk. Given that Morax is more efficiently used in Xiao and Itto teams, Thoma fills in a convenient gap for Hu Tao teams. The old school Hu Tao team (Hu Tao-Xingqiu-Morax) has now been replaced with new school variants (Hu Tao-Yelan-Thoma).
Thoma is not the first pyro shielder in the game. Xinyan exists too. And Bennett can also be used on Hu Tao teams to enable pyro swirl. It is just that Bennett's healing can make Hu Tao lose other buffs, but more importantly, Bennett is valuable on many other teams, so having someone like Thoma to replace Bennett can be convenient.
In the context of yuanshen/genshin, there is something interesting about Thoma's Chinese name Tuoma, specifically the Tuo. Considering that Thoma is also a housekeeper for the kamisato state, interacting with ayato and ayaka, the Tuo has additional significance as well. It reminds one of a certain Tuotuo who wrote the histories of the Liao, Western Xia, Jin, and Song dynasties before all of these Chinese kingdoms were merged into the Mongol Yuan.
E gives shield. Q gives fiery collapses. Passive increases fiery collapse damage based on his hp. Q lasts 15s (can increase to 18s with C2). The fiery collapses have ICD, but they can still be used for burgeon teams (reason is if they applied pyro to fast like Xiangling's Q, the enemy would have the pyro status most of the time, disabling the creation of dendro cores through dendro/hydro). Q costs 80 energy so may want to stack some ER.
C6 buffs NA/CA/PA damage by 15%.
Thoma has two main uses: 1) as a general shielder, specifically for Hu Tao, and 2) as a burgeon driver.
As a shieldbot, just stack full hp with tenacity and noblesse. You can even use an R5 black tassel or favonius. To build more damage, one can go for hp/atk sands, pyro, crit.
As a burgeon driver, one can stack full EM, or do mixed ER/EM. For weapons, use the dragonbane (EM), favonius, or even scarlet sands. For artifacts, use gilded (EM).
How much EM is enough? 500 is a good start to make the burgeon do decent damage. Some builds can easily reach 1k EM. And with Nahida's Q and other EM buffs, more is possible.
Note for burgeon he really does want ER to spam his burst.
For Oven/Burgeon, Furina and Yelan/XQ may result in some differences in seed generation given Furina has slower hydro app. Furina is mentioned because she can be more convenient as an E applicator whereas Yelan/XQ require more energy, while Thoma also has high energy requirements, resulting in possibly high ER requirements for a Thoma-Yelan/XQ team compared to a Thoma-Furina team.
In general burgeon drivers do not want to apply pyro too fast like Xiangling. Enough time needs to begin to apply dendro and hydro to create the cores before detonating them with pyro. That is why Thoma Q and Dehya E can work as burgeon drivers.
Note a pyro attack may not apply pyro to the enemy but can still proc burgeon. However, burgeon, hyperbloom, and bloom are limited by damage sequence to only a finite number of instances of damage within a second.
Among transformative reactions, burgeon and hyperbloom have the highest scalings with EM. In fact they all deal the same damage for the same EM, everything else equal. Burgeon does an aoe dendro explosion, hyperbloom creates dendro sprawling shots that home in on targets, and overloaded does an aoe pyro explosion. These reactions are affected by res shred (for dendro and pyro).
For hyperbloom, there are many electro drivers including Cyno, Raiden, Yae, Fischl, etc. For burgeon, there is Dehya, Thoma, and even Diluc to some extent. But what about overloaded? Overloaded would need constant pyro and electro application. The burning reaction is one way to sustain the pyro status. So an example overloaded team would be something like Dehya-Nahida-Cyno, where Dehya-Nahida sustain burning, while Cyno constantly procs overloaded with his electro attacks.