Tanaka is a cute! Also fuck off reddit.

Who is Tanaka? Well, if you've read the manga and you're not a secondary, you already know. [spoiler]She's the Ether Liner or Knight Liner.[/spoiler] Here's why.

Temperature. She's really hot. She's piping hot. She's smoking hot and not only in a lewd way. Servants don't emanate much heat by themselves, Most of their fire-based abilities are based on their mana. Illya, Kuro, Bazett, ko-Gil and even Julian would notice any traces of mana, but they didn't. So she's not a Servant, but we'll get to that later.

Toughness. She tanked a direct hit from Mjolnir without any defensive NPs or armor and only recieved minor injuries. Mjolnir was basically no different from a street pole to her.

Sleepiness. It's also an important part about her. She's a sleepyhead, but not only in a moe way. She decided to take a nap after Beatrice made a pincushion out of her torso with fake NPs and then in mere minutes or even seconds she fully recovered and tanked that Mjolnir hit easily.

Memories. She doesn't remember much about her past but we can assume that she's been experimented on (it wass dark, foggy and gloomy, she was always alone, she remembered what pain is only after Illya said that word to Beatrice). But enough assumptions, here're facts. She's from the Miyuverse and it's obvious. She thinks that winter is summer and spring is autumn (and Kotomine indirectly confirms that's the way Miyuverse works). She also never heard about any people in the city. But she remember that Miyu is captured by the Ainsworth and that she must destroy them. But not Angelica. Was she expecting her (but not Beatrice) to be an ally all along? Also, there's another thing. She said that she hates Illya being hurt. But with Tanaka around Illya was never hurt. Another funny thing about her memory or rather her mind in general is that she doesn't know much about things, but she can analyze materials well even by their smell, just like she did with Kotomine's cooking twice.

Ainsworth. There're 3 important facts about them and they're related to Tanaka as well. First of all, Julian didn't know who Tanaka is (or at least until Erika told him). He was surprised when she called him a "Counterfeiter". But why? Well, here's why. Darius-Julian used 2 NPs: Apneic Beauty and Authoritarian Personalism. ko-Gil claimed that he never heard about them. Why? Because they do not originate from the Gate of Babylon treasury. So they're too "modern" for him, like Excalibur, right? Wrong. Kuro doesn't know anything about them (except the general concept) either. What if they're from the future? There's a huge pile of rubbish NPs and Grails in the Ainsworth castle, but what if someone else managed to create working NPs? A guy who replicates stuff from the past and a guy who replicates stuff from the future. Like pottery. The final thing is the huge black cube of mud which can create Servants. We don't know what it is except that it's Julian's trump card (and he's probably going to utilize it in Illyaverse soon), but Tanaka knows that it's his trump card as well, that's why she incinerates all the Servants and with Illya's help forces the big cube to shatter and shrink. [spoiler]Also, ko-Gil is a cheeky bugger. He casually talked about his visit to the Ainsworths with Kotomine. They're clearly hiding something, probably it's the continuation of the Sakura's fake Gilgamesh card story.[/spoiler]

World. The main danger to the world according to ko-Gil and Angelica's info dump is "something unlike mana, an entirely unknown particle", a "poison meant to wipe out everything alive". And here's the description of Grain from Notes:
The general term given to the toxic particles overflowing in the Earth of Over Count 1999, after the world lost its capacities to sustain life. Also called “ether” because even though poisonous, it promoted the surface of mutated life forms: the A-Rays, Liners and the Ether Liners.

Knight Arm. Here's the main reason why Tanaka is a Knight Liner.
Liners are simply humans with the ability to live in an environment that is abundant with Grain.Though Liners are mostly inferior to the A-Rays, the development of Ether Liners (knights) balanced out their combative forces. The "Knights" are Liners that were strongly affected by the abnormal environment of the dead planet, having Grain crystallize in their bodies as if it were bone. After reaching maturity, the Grain is released into a definite form outside their bodies. These masses of Grain become known as Knight Arms (Demonic Swords), which have a variety of effects and allow them to fight against A-Rays without any support. The number of people who obtain a true Demonic Sword, one powerful enough to be called a weapon, is small, and their numbers only total seventy-eight.
So, it's exactly like her arm. Her arm is her armament - a detachable sword (hell it even has fingers with phalanxes on the blade).
Here's another thing why she can't be a Servant. First of all, she doesn't have a Master. She's also not running on mana. Why would she if this world's mana is already withering away?
Tanaka hates Erika as a competitor. What if she meant competition between liners (if Erika became a liner or fake Liner when she turned into a Dark Erika) and not just 2 awkward moe ladies? Though that one is a pure speculation. Also, Erika said that "the world's "truth" passes judgement on the counterfeits, on us". Don't know how to interpret that one yet. Tanaka is either the world's "truth" or its judgement. Tanaka also called Darius-Julian a counterfeiter. Pretty similar to faker, don't you think so?

Series' title. Fate/kaleid liner Prisma☆Illya has "liner" in it. Dixi. Well, not really. We can break down the whole thing. Fate/ refers to the setting and the whole first season and corresponding manga chapters. Kaleid is Kaleidoscope (the Second Magic Miyu and Illya used to summon 9 Excaliburs from different worlds). Also Kaleidosticks Ruby and Sapphire. Liner is a Liner and Tanaka is Tanaka. No idea about Prisma (Julian's black cube? unlikely) and ☆ (though it's a symbol on Ruby), so they're probably reserved for the future.