Sweets Heroic Spirit

The self-appointed title of Cas-ko (Blue Caster).
Forget about hiding your true name, or proving the existence of your soul, or becoming your Master’s faithful weapon.
It is a heroic spirit who lives for amusement and fun in the name of romance, love, and happiness.
To Cas-ko, surviving to the end and winning the Holy Grail War is no more than a bonus, and it would appear that her true mission is to bring happiness and good fortune to her master.

Fate/EXTRA Material: Encyclopedia of Fate/EXTRA