Freezing

A prequel to the original series, the story centers on Chiffon Fairchild's first year at West Genetics prior to her becoming Student Council President, and also explains the origin of her title, "The Unmatched Smiling Monster".
(Source: Wikipedia)

The story centers on Kazuha Aoi's second year at West Genetics and Yumi and Elize's third year at West Genetics as part of the elite Numbers unit.
(Source: Wikipedia)

Short stories included in the "Freezing Magazine" bundled with the first press release of the Blu-Ray/DVD volumes of the Freezing anime.

The Freezing Pair Love Stories manga tells the story of how the various Pandoras and Limiters met.
(Source: Anime News Network)