Sasaki Kojiro

The word itself is Legend with a japenese accent thus pronounced Regend as for how he got it is due to the large number of rider(dragons) enemies in France and thus Sasaki being extremely useful at that part of the game in spite of being a 1*. Add to this some guy soloing Fafnir with him and you have the R(l)egend of the Dragonslayer Sasaki Kojiro helped by the meme gaining enough prominence that we got a "Dragons aren't so different to swallows" line from Kojiro in an event(Don't recall which one).

He's a 1 star servant capable of absolutely destroying "oversized swallows"

Just like Nakara3 said,
Dragonslayer.
You dont know how frustrated and hopeless when was a noob playing Chapter 1 but facing multiple Fafnir.
But, wait, we have free Kojiro!

I could be wrong but if I recall there was a misspelling in his interlude quest back when it first launch that read "Regend of Samurai" or something like that. They eventually corrected the misspelling to "Legend of Samurai" but the name stuck and he got the nickname of "Regend".

That's what I've heard at least.