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The Witch of Symbols, presumably known as 10^21 (10^21) is a witch from the mobile game Magia Record. Currently in-game, its existence was only known through it's familiars, the Echos, Until Arc 3 when it was revealed.

In the anime for Magia Record, a large Familiar with a green hue was seen within the Symbol Witch's barrier. Upon its defeat, the barrier disappeared. This is most likely the final form of the familiars as they grow through the different stages.

Witch

象徴の魔女　その性質は『啓示』抗えない使命を帯びているかのように、ひたすら繁殖を続ける魔女。意識を持たないので、出自や年齢はわからず、何をしたいのかも考えたことがない。ただ、自分を増やしたいという空疎な行動原理に従って、今まで増やしてきた自分の分身と連携しながら更なる分裂を続けている。しかし、繁殖以外の意思がないので、いくら数を増やしても、見つかると簡単に潰されてしまう。ゆえに魔女にとっての栄華はほど遠い。

10^21
Runes	10^21
Type	Symbol Witch
Nature	Revelation
Episodes	Puella Magi Madoka Magica Side Story: Magia Record

As if charged with an irresistible mission, she is a witch who continues to reproduce ceaselessly. Since she has no consciousness, she does not know her origin or age, and she has never thought about what she wants to do. However, according to an empty behavioral principle of wanting to increase her own number, she continues to divide further in cooperation with her alter egos that she has increased up to now. However, since they have no other intention than to reproduce, no matter how many they increase in number, they are easily pickled when they are discovered. Hence, prosperity is far from attainable for the witches.

Minions

人間から奪った生命力に応じ成長する姿を変えると言われる象徴の魔女の手下達の中で、イス憑きと呼ばれる種類他魔女の手下に比べごく僅かな愉力を元に樣々な姿へと何段階も成長する。繁殖そのものを目的としており、ある程度成長した者は魔女にならずとも自身で新しい手下を産み落とすことが出来る。椅子を形どり、その利便性と完成されたフォルムを噛みしめて成長している模様。

Echo
Runes	ECHO
Type	Symbol witch's minion I-IV
Duty	Proliferate
Episodes	Magia Record, MagiReco Episode 6

Minions of the witch of symbols. Their role is to proliferate. These Familiars of the Symbol Witch are called Haunting Chairs. They grow and transform by absorbing vital energy from humans. Compared to other Familiars they can change shape with a small amount of vital energy. Their primary goal is reproduction. Once they have grown enough, they can produce other Familiars, despite not being Witches. They grow, thoroughly enjoying the convenience and completeness of their forms.

人間から奪った生命力に応じ成長する姿を変えると言われる象徴の魔女の手下達の中で、ヒト憑きと呼ばれる種類他魔女の手下に比べごく僅かな愉力を元に樣々な姿へと何段階も成長する。繁殖そのものを目的としており、ある程度成長した者は魔女にならずとも自身で新しい手下を産み落とすことが出来、1匹見かけたら30匹は存在すると恐れられる。見つけたら増える前に必ず駆除しよう。

Echo
Runes	ECHO
Type	Symbol witch's minion II-IV
Duty	Proliferate
Episodes	Magia Record, MagiReco Episode 7

人間から奪った生命力に応じ成長する姿を変えると言われる象徴の魔女の手下達の中で、トリ憑きと呼ばれる種類他魔女の手下に比べごく僅かな愉力を元に樣々な姿へと何段階も成長する。繁殖そのものを目的としており、ある程度成長した者は魔女にならずとも自身で新しい手下を産み落とすことが出来る。鳥を形どり、なんでも突き回しながら脳髄を揺らし、人類にはたどり着けない境地で成長している模様。

Echo
Runes	ECHO
Type	Symbol witch's minion II
Duty	Proliferate
Episodes	Magia Record,

Trivia

This witch's familiars and labyrinth have heavy feminine, fertility, and infertility symbolism.
- Her familiars grow from egg-shaped to being in cribs, then sitting in chairs (fertility), before finally becoming feminine humanoids with empty holes in their stomachs (infertility).
  - In the anime, the presumed final form of the familiars is humanoid figure standing on all fours, with an open hole where its head would be.
- The labyrinth features mannequins with pink broaches and rings (femininity), pink crystal pillars around which the rings sit and eggs rolling through the sky & across the ground (fertility), and dead, barren trees (infertility).
The entrance to her labyrinth is a keyhole (femininity and fertility).
Her name is presumed to be 10^21. The third Rune corresponds to the caret character (^). The fourth Rune resembles that of the number 2, but is not an exact match (the exact Rune for 2, as it is known to look, was used in the second season of the Magia Record anime adaptation). If it is indeed a new Rune, it is unclear what it corresponds to, but may be presumed to be a dash (-) or an underscore (_).
- 10^21 is the first name in the series that consists entirely out of numerals and punctuation marks.
  - It is one of three mathematical equations, or integers, based on the power of 10:
    - 10 to the power of 21 (or, the number 10 multiplied by itself 21 times). This would result in the number sextillion (1,000,000,000,000,000,000,000), whose name's prefix creates an English pun on reproduction.
    - 10 to the power of negative one (-1), resulting in 0.1.
    - 10 to the power of underscore one (_1), of unclear meaning. 10 to the power of just 1 results in 1.
  - Her head is the planet Mars (identifiable by the presence of the Valles Marineris canyon), named for the Roman god of war and consort of Venus, goddess of love and sexuality.
  - Her two halos (the rhombus/diamond above her head and the disc behind her), what appears to be a wing on her left side, and the large trumpet she wields, makes her resemble an Abrahamic angel.
  - Her body resembles a block of marble stone with parts of it carved out as if to create a statue or piece of art.
    - This, coupled with her name and the wording of her description, make her resemble a computer program or robot more than a living creature, and may indicate she is in some way an artificially created Witch.
An echo is the identical reflection of a sound off a surface, thus referencing that the Familiars are offshoots of the Witch that share her single-minded desire to reproduce into copies of her and each other.
- In Greek mythology, Echo was a nymph (a nature spirit) who was cursed for talking so much so as to only be able to repeat what others said. She later fell in love with a man named Narcisuss, who spurned her love and was thus himself cursed to fall in love with his reflection so intensely that he ultimately wasted away and died from not leaving his own sight. Eventually Echo faded until all that was left was her repeating voice.
- Two of the new Echo variants introduced in Arc 3 reuse designs seen in Episode 7 of the game's anime adaptation. There, they were released from inside one of Alina Gray's weapon containment cubes and had presumably been modified by her artistic endeavors and efforts to make them stronger.