This is a story from when Reimu Hakurei was still small, and the flesh-eating youkai was still big. And also... a story about the unknown miko.
Osana Reimu (otherwise known as "Little Reimu") is an episodic sort-of Film Comic created by Joyful (ジョイフル氏 ) of Nico Nico Douga. Set a few years before the exploits of Reimu Hakurei, this TouhouFan Fic deals with the lives of a younger Reimu, her mother as the current Hakurei miko, and Rumia - who, going by the popular Fanon that she is Sealed Evil in a Can in present-day Gensokyo, is currently an adult in this timeline. Despite knowing that Rumia is a youkai and that Reimu and her mother are humans, the three of them have a very strong friendship, until one fateful day when the harsh realities of Gensokyo and Rumia's needs as a youkai catch up with them...The sequel, Touhou Reireimu, returns to the present day where Reimu works as the Hakurei miko. Despite Reimu's usual attitude towards others, she is still considered a friend by her fellow humans and the resident youkai of the Forest of Magic, namely Rumia, who is now the younger, happier youkai we all remember. Things start out relatively normal until Rumia's seal breaks, forcing Gensokyo to deal with the aftermath of the events of Osana Reimu...Other than a few blatant Fanon elements (with the "Rumia being a sealed greater youkai" thing being the worst offender, which is the entire point of the series anyway), Osana Reimu and its sequel are both very faithful to the Touhou canon and its characters, and is arguably one of the most popular Touhou fan stories out there. English translations of both Osana Reimu and Reireimu (with the last episode of Reireimu still under production) are up on YouTube.Oh yeah, have some tissues handy.Needs Wiki Magic Love.
Tropes for the series as a whole:
Ascended Fanon: At its core the series is about the EX Rumia fanon, though it is nicely worked into a much bigger story.
Cerebus Roller Coaster: The beginnings of both stories are light-hearted and wacky, as you'd expect from Touhou. When shit hits the fan however, it makes damn sure you know it.
Film Comic: A notably impressive one; rather than using purely computer-generated graphics or pre-existing backdrops, the characters, backgrounds and even the speech bubbles all use the same hand-drawn art style.
Beneath the Mask: Turns out that Reimu's mom has a good reason for wearing all those masks. It's because she's hated and feared by humans and youkai alike, and wears the mask to escape their terrified/condescending stares.
Covers Always Lie: Despite being named Little Reimu, her mother and Rumia are given just as much focus as the title character. The Tagline also only alludes to Reimu and Rumia until the epilogue.
Creative Closing Credits: Each episode (or part of the episode if it's a multiparter) showcases a large number of fan creations based on the series during the credits. These range anywhere from hand-drawn art, to some amazing digital art, and even to some physical artworks like a hand-made paper mache of one of Reimu's mother's masks.
Curb-Stomp Battle: The first meeting between Rumia and Reimu's mother before becoming friends resulted in the former getting utterly ruined. The second fight is more even, though this is only because Reimu's mother was trying to lose on purpose.
Dark Is Not Evil: Rumia is more of a loner trying to survive than outright malicious. Reimu's mother also has the power to draw upon her own inner darkness for a huge power boost.
The Dreaded: Reimu's mother is an amazingly powerful miko. Unfortunately, she's seen as a walking terror rather than a hero by both humans and youkai.
Grumpy Bear: Rumia was an awfully disagreeable person before meeting the Hakureis.
Harsher in Hindsight: In established Touhou canon, Reimu, while preferring not to be bothered, really hates that she isn't recognized enough as a miko and doesn't get enough worshipers for her god at her shrine. If this fanfic is to be believed, then the alternative is much, much worse.
Heroic Sacrifice: Reimu's mother pulls an incredibly difficult one, going as far as tricking Yukari and lying to Rumia so that the lattermost will eat her to avoid death by starvation.
Shoot the Shaggy Dog: Averted. Yukari is about to do this to Sealed!Rumia in retaliation for killing Reimu's mother, but because her final wish was to let Rumia live and since this Rumia no longer posed a threat to anyone, Yukari decided to let her go.
Used to Be a Sweet Kid: Young Reimu is adorable. Definitely a far cry from the apathetic and lazy-ish shrine maiden we know today.
Villainous Breakdown: YMMV on the "villainous" part, but Yukari is driven up the wall with how out of control Rumia ends up being. Reimu's mother knows that Yukari can't use gaps on Rumia (since Rumia, a living void of darkness, has no spiritual boundary for Yukari to mess with), Rumia utterly refuses to eat humans again despite Yukari's Breaking Speech, Yukari can't simply kill Rumia because this would rob Gensokyo of its nighttime, and the inability to do anything about Rumia ends up extending to her friends - Yukari is ultimately unable to stop Reimu from confronting Rumia in the sequel despite her best efforts.
Tropes for Touhou Reireimu:
Art Evolution: The fight scenes have been improved over the prequel's.
Ax-Crazy / Laughing Mad: Awakened!Rumia. Suddenly remembering the circumstances of her sealing just breaks her.
Dissonant Serenity: Rumia acts strangely calm before Reimu engages her. She doesn't keep it up for long though.
The End of the World as We Know It: When Rumia wakes up from being sealed, her darkness rapidly starts to expand and dissolve Gensokyo. Everyone thinks it's because Rumia isn't aware of herself at the time, but as Reimu finds out, the spreading black death ball is quite intentional.
Fanon Discontinuity: Concepts vary on what EX Rumia's One-Winged Angel form should look like, but most of them seem to go with: black feathered wings, a halo, a BFS, and happening immediately upon the seal breaking, but otherwise looking exactly like vanilla Rumia. Do some of these sound cliché? Joyful thought so, and decided to put a new spin on these concepts for a more monstrous design: the wings are now demonic with thin webbing and bone-like structuring, the halo is broken, the BFS was taken out completely, her body is covered from head-to-toe in darkness except for Glowing Eyes of Doom, and the transformation only happens when Awakened!Rumia wants to make it clear that she is trying to kill Reimu. The revamp was met with many positive reviews.
Friendly War: The "sustenance without killing" aspect of the Spell Card System is an important plot point, used by Reimu to convince Rumia that Gensokyo has changed from the cycle of killing it used to be.
Ghost Memory: Apparently when Rumia eats a human, she not only consumes their inner darkness, but the memories that were the root of that darkness as well, meaning that she vividly sees all of the pain and sorrow Reimu's mother suffered through. It's so much that it motivates her to destroy Gensokyo in retaliation.
Heart Is an Awesome Power: Reimu's "Fantasy Nature" puts a literal spin on this, drawing out what's inside of the Hakurei miko's heart and converting it into something they can use to fight... though Reimu hasn't ever used it successfully. This is because she has feigned apathy and lied about her feelings for so long that she doesn't even know what's in her own heart.
Offscreen Moment of Awesome: Reimu and Marisa have a pretty epic showdown with Yukari, judging from the state of the battlefield afterwards, and win. Unfortunately, this battle is cut off by the credits, and we only see the aftermath in The Stinger.
One-Winged Angel: This time it is Rumia, having gained the ability of Reimu's mother to cover herself in darkness.
Revenge: Given Yukari's more questionable actions, it's clear that she still holds a grudge against Awakened!Rumia.
Secret Keeper: Since she played the role of Memory Wiping Crew, Yukari is the only person who knows about the events of the prequel, keeping it secret so that Rumia can live a free life like Reimu's mother had wished.
Interestingly, Yuuka also remembers. Apparently Yukari tried to wipe her memory too, but this didn't pan out for obvious reasons. Fortunately she also keeps a lid on it for Rumia's sake.
Thou Shalt Not Kill: Invoked and demonstrated by Reimu. In a flashback pre-Spell Card System, Reimu admits to Yukari that she can never really get into youkai extermination. She covers it up by saying it's boring, but Yukari isn't fooled and all but points out that she understands that Reimu doesn't like the thought of killing things. This revelation sees Yukari encouraging Reimu to change the way things work in Gensokyo if she doesn't like them, prompting the creation of the Spell Card System.
Well-Intentioned Extremist: When Rumia's seal breaks, Yukari finally decides that enough and enough and attempts to have her completely exterminated before Gensokyo is destroyed. She even fights seriously against Reimu and Marisa to stop them from going after Rumia, believing that they're incapable of handling something of this scale...
Worf Had The Flu: ...and fails, Reimu and Marisa soundly defeating her. She also becomes completely exhausted after finally deciding to help Reimu reach Rumia. She chalks both cases up to old age.