Minamoto no Raikou

Regarded as the strongest mystery killer in Japan, Minamoto-no-Raikou is the leader of a group of powerful Heian period warriors known as the four heavenly kings. Raikou has been summoned to Chaldea as a berserker class servant that has pretty much adopted master as her child, always ready to smother them to the point that she's been associated with other unique (read:unhinged) heroic spirits the have been called the stalkers. Despite this, Raikou is still a kind and helpful servant (unless your an oni) who has cemented herself as a valuable servant whether it be on the battlefield, alongside the cooks, or ensuring public decency is kept as a shadow prefect (despite the fact that her prefect uniform is more revealing than a defective one way mirror and the bikini under it is just two piece of fabric on strings that barely cover her huge breasts.)