My thoughts on who's one of the most underrated touhou characters, of course, goes to THE disciple of Bishamonten herself, Shou Toramaru. Not to mention that as of now, she is the ONLY Stage 5 boss up to Legacy of Lunatic Kingdom who has not been a playable character, as every other Stage 5 bosses up to Legacy of Lunatic Kingdom has been a playable character in some fashion. (Sakuya in PCB, IaMP, IN, PoFV, SWR, HSTS, DDC, and UM, Youmu in IaMP, IN, PoFV, SWR, TD, and WBaWC, Reisen in PoFV, SWR, HSTS, ULiL, LoLK, and AoCF, Komachi (if you count her as a Stage 5 Boss) in PoFV and SWR, Sanae in UFO, HSTS, TD, LoLK, UM, and UDoALG, Rin in UDoALG, Futo in HM, ULiL, and AoCF, and Seija in ISC)
Personally, I think Shou is interesting because of her backstory, which is her manifesting as a "tiger youkai" because of the fear created in stories from Japan about tigers that had spread there since tigers aren't native to Japan and that her entire existence originally stemmed from these stories and the people of Japan remaining ignorant about what real tigers were like. Then she met Byakuren Hijiri back then when she was srill a famous monk in the Outside World and she scouted Shou to become a disciple of the Buddhist God of treasure, warfare, and punishing evil, Bishamonten, (since Byakuren was devoted to him and that she also found out he was feared by youkai because they thought he would exterminate them as well as was around the temple most of the time, causing youkai to be scared of going around the temple.) to gain the trust and the like of the youkai living on the mountain by introducing her to him since she was the most straightforward and honest youkai, and that prompted Bishamonten to not only quietly take Shou as his disciple and avatar, but also gave her his servant, Nazrin, to assist and watch over Shou. (And didn't cause any problems at all despite Bishamonten's worries.) Meanwhile too, she was also hiding her true identity as a youkai from the humans. As the years passed then, Shou proved herself a more-than-worthy disciple, and also had a more solid identity as Bishamonten's avatar, which also prevented Shou from fading away.
After Byakuren was sealed away into the depths of Makai in Hokkai because of her getting exposed of wanting to help the youkai instead of exterminating them such as her encounter with Murasa and was ostracized for being a, quote, "demon in human form" because of that up to her sealing, Shou couldn't do much, and if she had tried to do something, the fact that she was a youkai would've been revealed, so she continued serving Bishamonten and was eventually also sealed along with the other followers of Byakuren (Nazrin, Ichirin and Unzan, and Murasa.) along with the Palanquin Ship in the Underworld.
Fast forward hundreds of years later, the geysers that Utsuho created back in SA launches the Palanquin Ship out of the Underworld and onto the trajectory of Hokkai to rescue Byakuren from her seal using the Flying Storehouse and the Jeweled Pagoda of Bishamonten, but the Flying Storehouses fragments were scattered from the geysers and by Nue Houjuu (who was also sealed in the Underworld by herself since she was tired of watching humans get frightened and imagine all sorts of strange things from afar and went to sleep in the Underworld and was also released too.) as a prank where she placed the Seeds of Unknown Form into them, turning them into the UFOs we use in Undefined Fantastic Object in an attempt to impede the group's effort to unseal Byakuren and was also how Reimu, Marisa, and Sanae saw the fragments, and that Shou lost the Jeweled Pagoda of Bishamonten herself prior to the events of Undefined Fantastic Object, with her ordering Nazrin to find it.
Thankfully, though, thanks to the efforts of Reimu, Marisa, and Sanae collecting the disguised fragments of the Flying Storehouse much to the delight of Shou's group; Nazrin, Ichirin, and Murasa, and with Nazrin finding the Jeweled Pagoda of Bishamonten by Stage 5, Byakuren is successfully unsealed out of Hokkai by Reimu, Marisa, and Sanae, finally deciding to break the seal out of pure curiosity.
Everyone, including Byakuren, all finally leave Makai, and the residents of the Palanquin Ship (Nazrin, Ichirin and Unzan, Murasa, Shou, and the now unsealed Byakuren.) all fly around on the Palanquin Ship for a while, but eventually settle themselves near the
Human Village and renovate the ship into a temple with the help of Kanako Yasaka and Suwako Moriya, as Sanae had offered the Moriya Shrine's help in exchange for passage back out of Makai, (Since Sanae couldn't escape on her own.) which they name after Byakuren's deceased younger brother, Myouren Hijiri, creating the Myouren Temple. The temple then becomes popular with youkai, and eventually Nue realizes that something like this would benefit her as well, and joins them in living there.
Shou is SUPER underrated in my wholehearted opinion, and I at least somewhat hope that ZUN turns her playable in a future Touhou game.
