ShingetsutanTsukihime is a pretty expansive work - a main visual novel, the Kagetsu Tohya supplement, and the Melty Blood games. The last bit, Melty Blood, is especially jarring to a Type-Moon fan who is late to the party - there are many versions, and the format itself is very different from what is normal for Nasu works: a fighting game.
Melty Blood is the original game. The story supposedly follows Satsuki's route, which was never released with any of TM's games, which takes place one year after Tsukihime. The story and plot doesn't change with the subsequent expansion, but a lot of side stories are added (more Neco Arcs!). In the original Melty Blood, only six of 14 characters were available to be played, the other 8 had to be unlocked through playing Story Mode. This was later changed in an update for the game (aka the "Nero" patch), which unlocked all the characters from the start.
She never appears. She is only mentioned a total of three times in the full story, until ReACT. However, if you read into the game's story, her appearance could might as well been a fake created by Wallachia or White Len...
Melty Blood ReACT is an expansion for the original game (like Burning Crusade was to World of Warcraft). This expansion added 2 playable characters and 4 non-playable ones. I believe ReACT also adds an Arcade mode, which is a continuation of sorts of what happens in the Story Mode played out in fighting game style instead of visual novel style. Start with this game if you are just starting out with Melty Blood series and want the whole story (original Story Mode + Arcade Mode).
Melty Blood Act Cadenza is a further expansion of ReACT. This added 4 additional playable characters and various graphical and gameplay changes. Note that this version does NOT have a Story Mode.
Melty Blood Actress Again is a sequel to the previous game. It features many drasticly new mechanics, two new characters, and 3 different fighting styles for all known characters. Several secondary characters have had their storylines expanded upon to include what happened to them after the main storyline.
to answer the question in the comments:melty blood react final tuned -> actress again is the full storyline (of melty blood, melty blood itself being a sequel to tsukihime)melty blood original and react/react final tuned have the exact same story.
Sion is an Alchemist of Atlas, a magic organization in Egypt independent of the Mage's Association. According to her calculations, Tatari is going to manifest in Misaki. Bearing a grudge against the mysterious vampire, she came in an attempt to kill him. She quickly meets up with Shiki and commences the first battle of the story. Following that, she and Shiki track down the mysterious Tatari, but she eventually lies and says they have defeated him before they really have managed to do so. Shiki catches on and they eventually face Tatari in battle where, with the assistance of Arcueid, they finally manage to destroy him for real.
The reason for her grudge against the Tatari is rooted in a failed attempt by the Church to kill him at an earlier time. However, the rest of the group was killed, including her friend Riesbyfe Stridberg. Sion was only spared because Tatari realized she was a descendant of his, so he merely drank her blood. Due to the nature of the Tatari, she has yet to fully vampirize and intends to kill him before this can happen.
Vampire Sion/Sion TATARI is story-wise an Alternate Continuity where Sion has succumbed to her vampiric impulses and is on the verge of succeeding Wallachia as the new TATARI. Despite this she can still fight her normal self.
Alchemy Is Magic: "Mental" Alchemy, where the lineage is constantly upgrading their bloodline to have higher cognitive abilities. This also lets her do things like compartmentalize her brain processing or drastically overclock it in a pinch. Most Atlas alchemists use this for calculating possible futures in a similar vein to Laplace's Demon, with them obsessing over and documenting potential ends of the world. Traditional alchemists also exist. Alternate Continuity: Within Melty Blood itself, one can play as "V. Sion/Sion Tatari" and "Normal Sion." Unlike other Evil Counterparts who are just Tatari Nightmares, Vampire Sion is an Alternate Continuity where she succumbs to the bloodlust. Compare "Evil Ryu" from Street Fighter. In Fate/Grand Order, the end of the second Lostbelt Gtterdmmerung ends with the introduction of Sion Eltnam Sokaris, since Grand Order is a world where she did not become head of Atlas. Assist Character: As Vampire Sion, she can summon copies of Satsuki, Ciel and Arcueid to attack. Awesomeness by Analysis: Alchemists of Atlas are even capable of predicting the future because of this. It's basically a form of Laplace's Demon. Ax-Crazy: Kinda. Since Re-Act, Vampire Sion picks up her parent vampire's habit of shouting Katto during battle. It's not as extreme as his case, but hey, she still has to grow into the role... Defrosting Ice Queen: Courtesy of Shiki. She goes from The Stoic to Shrinking Violet in his presence. Dirty Mind-Reading: At one point in the manga Shiki politely, with a friendly smile asks her what he is thinking right now. After a few seconds she blushes as red as a tomato, and angrily lashes out and hits him. Sadly, readers weren't told what was he thinking, or if Sion was involved. Handguns: Sion is the only Melty Blood character to carry a ballistic gun. Specifically, it's the "Barrel Replica", a copy of the Black Barrel, one of the Atlas Institute's seven superweapons, which is apparently designed to look like a regular CZ 75. She will run out of bullets after 13 shots and need to reload if you use it too much. Hidden Buxom: She's actually an E cup, although its not very visible. Hyper-Awareness: Sion can "overclock her brain" through her techniques and predict probable future outcomes using math and magic. I Hate You, Vampire Dad: She's out to kill Tatari for, not just wiping out the group pursuing him, but also afflicting her with vampirism. She's running on borrowed time before she succumbs to bloodlust, however. Kiss of the Vampire: In Melty Blood, Vampire Sion can use this as a Grapple Move as well as heal herself this way. Meaningful Name: In Japanese, her name is pronounced shion. That's the name of a flower associated with memories. Sion can read and manipulate memories using her Etherlite. Meta Guy: In the English translation of Under Night In-Birth, Eltnum runs with it, often commenting on Melty Blood. Not So Stoic: Shiki you womanizer... In her winquote against Shiki, Sion comments that she had to fight because he distracted her from her calculations, but it wasn't his Mystic Eyes of Death Perception, it was... just him. She also very nearly breaks down when she realizes that her best friend has been brought Back from the Dead... And is stuck serving the villain. Odd Friendship: Sion was good friends with Riesbyfe Stridberg when ordinarily, mages and the Church aren't on good terms. In Self-Deprecation humor, the Japanese Nasuverse dictionaries comment that unlike the other females in the series. she's truly a rare personality who can get along with anybody. Pinball Projectile: Half-Moon-Sion can do this. While it's acceptable that the bullets may ricochet when shooting at the ground if one stretches their suspension of disbelief for a bit, one can only wonder how the bullets ricochet on the top of the screen. Power Up Letdown: Lore-wise, it would seem Vampire Sion is stronger; however, her lack of her gun and Etherlite whip techniques severely limit her options, making her much easier to deal with than her regular self. Psychic Link: Etherlite is also capable of doing this. Sion's family are literally nicknamed "spiritual hackers" because of this. Resist the Beast: Several of V. Sion's pre-fight quotes are pained pleas for the opponent to run away. Significant Anagram: Her family name of 'Eltnam' can be rearranged to form the word 'Mental', which is the school of Alchemy she practices. Split Personality: A really unique twist on this. She has multiple "rooms of thought" while most people have one, so she thinks of up to at least seven subjects at a time. It's the same personality, but it's thinking about seven different things at once with the same focus. Teen Genius: Thanks to her ability to make millions of calculations in a fraction of second.In one of her Act Cadenza winquotes, Sion comments that "Hand-to-hand combat isn't so bad. When I return I may add it to my curriculum", suggesting that she didn't receive any formal training, but thanks to her ability, she's perfectly capable of incorporating close combat whenever necessary. Transplant: Bordering on expy, "Eltnum" appears in Under Night In-Birth with the same background as Sion. It's notable all of Eltnum's sprites were made for an HD version of Melty Blood, but ironically even as an HD version was made in conjunction with the Tsukihime remake's release, the plot of Type Lumina means that she won't be making an appearance, leaving Sion's current sole HD appearance in Under Night In-Birth.
Miyako is Shiki's much younger sister from the Arima family. She rarely talked with Shiki, so he assumed she didn't like him. In fact, the opposite was true: she loved him very much, but was shy so she couldn't approach him. Instead, she would get embarrassed, tackle him, and then run away. She views the Tohno family as having 'kidnapped' Shiki and goes to 'rescue' him. She has a fascination with Chinese martial arts, and Wallachia increased her fighting ability. She was mentioned briefly in the original Tsukihime and Len was mistaken for her in Kagetsu Tohya, but she did not receive a proper introduction until Melty Blood.
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