Ritsuka Fujimaru wasn't anyone special. She was just a regular girl that came from a regular family. No talent to speak of. She was the epitome of average. And yet, she was their hero.
Heroic spirits in basic terms are people who have done extraordinary things when they were alive. Whether it be good or evil. Fron the legend of the Knights of the round table, to the exploits of Nobunaga Oda in the Far East. History given form, Heroes given life. And they all give their absolute Loyalty to her.
Why? Why does Heroes from different times absolutely adore her, respect her, protect her with every fiber of their being. There is only one reason.
She is normal.
She wasn't normal in a sense that she doesn't have any wierd quirks. She was quite a pervert enjoying peeping on all the beautiful women of Chaldea, groping them without any hesitation, calling them her 'waifu harem' or something. pranking the guys, messing with them all. And yet for how annoying she was, her servants just find her more endearing.
You ask why she's 'normal'?
She's normal in a sense that she never treated them as figures to be worshipped. To be prayed, to be feared. Ritsuka Fujimaru treated her servants as normal people.
Ritsuka Fujimaru to other people may be just some random girl. But to her servants, she is their greatest hero.