mudflaparts:

Runes: G A R N E T
Type: Fusion Witch
Nature: Devotion

The Fusion Witch. Her nature is devotion. The amalgamation of two kindred spirits that pledged to never be without the other, the desire to be forever unified is her only driving force. Her barrier intends to trap and consume all those who would threaten to tear her halves apart. 

From my Puella Crystal Magi au 

Ruby and Sapphire had similar wishes when they became puella magi; both were victims of extreme isolation, longing to never be alone again. After making the contract, the two puella magi found eachother and had been living and fighting together ever since. The very thought of being in a world without the other is more than enough to send them over the brink of despair.