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. Traditional alchemists also exist.
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.
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.
"I don't care for whatever plans you have for humanity, just give Onii-chan back!"
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.
Anime Chinese Girl: She has the clothes and knows martial arts, but she's not actually Chinese.
Clap Your Hands If You Believe: In one of the stranger original arcs in Melty Blood, the Tatari tried to focus on Miyako's fears. But something was radically different about her "pure heart" and instead of a temporary materialization, the "Influence of the Tatari" gave her Arcade-style KungFu. Her first impression of Kohaku was "Mad Scientist/Witch."
Sheathe Your Sword: Expels Tatari in the Melty Blood Act 2 Manga by getting more upset that Shiki starts to fight back (and hits her lightly) and runs away crying instead of escalating the fight.
Shout Out: The style of Kung Fu that she uses is from the King of Fighters games which is another indicator of the "fictional" origin of them. Fans have run with this, drawing pictures of her posing with the King of Fighters cast.
Red Arcueid is a hypothetical Arceuid that has given into her blood lust created by Wallachia. However, because Arcueid is such a powerful being, she is limited to one third of her normal power. Red Arcueid is completely bloodthirsty and insane with absolutely no value for life.
For the Evulz: Rather than go straight to killing Arcueid and cementing her existence, Red Arcueid decides to take her time and mosey across the city, having fun and messing with whomever she happens across on the way.
Neko-Arc was originally nothing more than a chibified Arcueid that would appear to harass Ciel in the Tsukihime hint corner Teach Me Ciel-Sensei. This trend continued into Kagetsu Tohya, but in Melty Blood she became a fully fledged character. As you can probably guess from her appearance and origins, she is not the most serious character around. In Carnival Phantasm, she is the owner and main bartender of the Ahnenerbe Cafe.
Beam Spam: Entirely possible for a player who's got fast-ass thumbs.
This is pretty much the entire reason she exists. As mentioned below, she started as Ciel's foil in the "Teach Me Ciel-Sensei" scenes, which directly addressed the player and had characters commenting on events happening in the game.
Mecha Hisui is a robotic version of Hisui created by Kohaku, most likely while under the influence of Wallachia. Her duty seems to be to guard the Tohno Mansion from intruders, of which there are a large amount. It's revealed that Kohaku had built an entire factory that mass produces these units.
Insistent Terminology: Kohaku is addressed by Mecha-Hisui as either 'Doctor' or 'Doctor Kohaku'. (And 'Nee-san'.)
Magic Powered Pseudoscience: What exactly Mecha-Hisui is made of isn't quite known but she was created "under the Tatari's Influence." It is highly unlikely that Kohaku, no matter how twistedly smart she is, knows traditional electronics, programming, and robotics to the degree necessary.
In the pre-battle dialogue with Aoko, Aoko refers to Mecha-Hisui as a magic doll.
Riesbyfe was a Church Executor and a close friend of Sion before dying at the hands of the Night of Wallachia in the incident that infected Sion with vampirism. However, Wallachia absorbed her information, also known as her memories and personality. Sion was able to steal this from Wallachia and save the information in 2 of the partitions of her brain and then destroy him using a bullet she created from both Riesbyfe's and her own Empathic Weapon. Later in the alternate future, Dust of Osiris used her timeline's version of that same information to recreate or copy her as a "Defense System Physical Spirit", which she brought with her back in time. Riesbyfe's story arc takes place in the Vampire Sion timeline.
Back from the Dead: Sort of. In Actress Again The Dust Of Osiris animated her existence and in Sion's good ending and Satsuki's ending, Sion sets it up so Ries's continued existence is fed by Sion's mana instead. What Time Paradox?
Sion had already captured her data from Wallachia. She used a combination of the program Dust of Osiris made and her own.
Downer Ending: Her story ends in one. Having taken place during the Sion TATARI continuity, Riesbyfe is forced to take her down. In the end, Riesbyfe defeats Dust of Osiris, the person who resurrected her. Osiris tells her how she instigated the new TATARI in order to save her. After a heartfelt talk between the two of them, the two die together as TATARI collapses.
Expy: Not particularly her character, but her existence is very similar to the Servants of Fate/stay night. She's a Defense System Physical Spirit created from the previous Tatari's perfect information of her after Oberon ate her.
In the manga, she even boasts that she is Sion's Shield; a similar oath to the one that Saber made.
Fan Nickname: Roast Beef Stroganoff/Strudelburger/Stringburger.
Posthumous Character: She was member of the Church and mentioned as a companion of Sion's in the group that was sent to try to stop Tatari, and died trying to buy her time to escape. Sion was extremely close to her, and kept a memento of her at all times. Then she appears in Actress Again, reanimated by the Dust of Osiris.
"I see... you're definitely some kind of imitation." [said to Shiki Tohno]
Shiki Ryougi from Kara no Kyoukai makes a specialappearance in Melty Blood: Actress Again. For a proper writeup, please consult the Kara no Kyoukaicharacters page. Shiki is explicitly stated in Melty Blood to have come from another universe, most likely because she and Tohno Shiki do not exist in the same universes. She was apparently brought there by Archetype:Earth, and her story mode ends with the two of them starting to fight.
Archetype Earth is the original form of Arcueid and the template for the vampires for planet Earth (thus an archetype, rather than Type like Brunestud and ORT below. However, in Current Code, she doesn't actually exist as a separate character despite being playable, merely as Arcueid's personality.
"Spin the vermin, reverse the guilt. Reverse the birth, reverse the world. Spin spin spinspinspinSPIN~~~!"
The Tatari is one of the more bizarre Dead Apostles out there and the thirteenth Dead Apostle Ancestor. While not particularly powerful on his own, the nature of his existence makes him almost impossible to track down and completely impossible to destroy through normal means. The reason is that in one sense there is no Tatari at all, only a recurring phenomenon on par with a hurricane. It merely appears for a single night every few decades. The name Night of Wallachia comes from one of its earlier manifestations not long after the death of Vlad the Impaler, who was already falsely being thought of as a vampire. Due to the Tatari's nature of manifesting itself as the fears of the populace, it appeared as what Vlad would be if he really were Dracula and slaughtered the village.
Due to her grudge against him for both killing her friend Riesbyfe and partially vampirizing her, Sion has managed to track down where he is next to appear, thus setting in motion the events of Melty Blood. It is said that Tatari will not die until the crimson moon once again appears in the sky, which will not occur for another thousand years.
Although he has a number of fates in the original Melty Blood, canonically the Night of Wallachia ends abruptly when Arcueid channels Crimson Moon Brunestud, announces that even in a thousand years Zepia will not succeed in making his miracle, and pulls the crimson moon from the future to turn him back into his vulnerable vampire form. He is then killed by Sion and Shiki. Despite this, due to the meddling of Aoko and White Len he appears as an avatar of his former self in later Melty Blood games, though he is still dead.
Dracula: The game does in fact point out that Vlad Tepes was a mortal; but one of the first massacres that the Tatari did in the past was a town in the Romanian province of Wallachia that feared the legend of the Son of the Dragon.
Amusingly the real Vlad Tepes appears as a Servant in Fate EXTRA and another interpretation in Fate Apocrypha; one wonders what would happen if the they encountered each other.
Enemy Without: He does not have a true form, he simply takes form according to the dominant fears and rumors in his "hunting grounds".
The fact that he does this is the reason for Red (Bousou/Berserk) Arcueid, stabby-mode Nanaya Shiki, Mecha Hisui, Magical Amber Kohaku, and possibly also Vermillion (Kurenai Seki Shuu) Akiha existing.
And hilariously Trevor Belmont in Castlevania: Curse of Darkness.
Jackass Genie: Hey, he just materializes as people's wishes. If people want a bumper crop, dead bodies for fertilizer. People want to be friends? Running in terror from him before they die is a great way to connect. People want the favor of their god? He'll become their god and favor them when they die for him. Really, he interprets every single wish as demand for a bloodthirsty rampage.
Not So Different: According to his warped logic, this is why he made Sion into a vampire.
Nigh Invulnerable: It takes something really special that Shiki can't just hit his Death Spot and kill him.
That may be a technical point. Shiki's power doesn't work on imaginary/weirdly-existing beings, hence Nasu's clarification that he wouldn't be able to kill a Servant (therefore proving that the famous crossover doujin is on crack and then some... sigh). He's literally described as a phenomenon, so killing him would be like killing a storm, or an earthquake... Maybe. ...Goddamn it Nasu!
To clarify, Shiki cannot kill beings outside of the Gaian cycle of life, as he himself is one of those beings. It's the reason why he can see death spots on living beings but lines on inorganic beings. It's easiest for him to comprehend death in beings most similar to himself, although he can still see death for anything under Gaian reality. TYPEs (the ultimate beings of each planet), beings like TATARI, and assumedly Servants, have a different concept of absolute death (Servants can die, but since they're stored in the Throne of Heroes, unless he can kill the record itself, Shiki can't "kill" them).
You get a full screen of this when you pull off his super damn magnum epic attack in Melty Blood. You will fail to not scream. TRUST ME.
Well-Intentioned Extremist: His final speech to Sion reveals that, after tracing out countless hopeless scenarios of humanity's self-destruction, he became the Tatari in hopes of eventually creating a variable that could alter the future.
You Can't Fight Fate: Despite his best efforts, he never found a way to even stave off humanity's extinction let alone stop it.
White Len is a version of Len created by Aoko Aozaki from the remains of Wallachia after he was defeated. She is said to made up of unused parts of the original Len, including both actually speaking and many of her repressed unhealthy emotions.