Weapon Stories

- What is this place? -
Weapon stories have been a part of the series since its first incarnation. As such, it would only make sense that Nier: Reincarnation also have this crucial element, as well. In fact, it's one of the main themes of the game, as each main character goes through the cage to repair memories in a four-act structure and restore a character's representative weapon.
Like the other games, stories are revealed as the weapon is upgraded. Some are directly related to the characters present in the game and the worlds they live in, while others are seemingly unrelated. Either way, the tales contained are worth a read.
You can read the weapon stories either on ReIn Guide, this playlist by Jaye Bird, or Nier Reincarnation Library on Tumblr. A playlist in Japanese is here.
You can also read about the weapons' gameplay utility in ReIn here.
The weapons come in three forms of "rarity", which determines its overall power. The rarities are 2-star, 3-star, and 4-star. Gems (or another material, depending on the type of weapon) can be used to raise the rarity of the weapon, and 4-star weapons can be raised to 5-star. Each weapon takes a set of materials that can upgrade the skills and abilities. There are also several more mechanics that are dependent on the type of weapon that can be found in the link above.