@HYBRID_BEING
Some necessary information from the background of the main story:
The Legendary Heroes were summoned in the past as well but were different people.
There were most likely other Shield Heroes before Naofumi
The legend of the Shield Hero is tied with the demi-human and their religious beliefs which is why they worship the Shield Hero.
This is the reason why they were arguing in this chapter and why Raphtalia knows about the legend of the Shield Hero.
Raphtalia does not know about Naofumi yet until
the rising of the next Shield Hero but probably thinks he might be cool because of the legends passed down of the previous Shield Hero