Who is Leilah in Tales of Zestiria the X? I am watching Tales of Zestiria the X and I didn't understand if Leilah the Seraphim is the soul of the sword or it's kind of the guardian of that sword that can strengthen it.

Who is she?
After having played to the video games what we learn about Leilah is that she is living in the sword waiting for a new "Dôshi" to come.

So Leilah isn't the soul of the sword or here to make it stronger. She's here awaiting a new "Dôshi" and that's why she ask to Sorey why he wants power.

When he answer her right she allows him to use the sword and become is Seraphin.

I would add that I don't think Seraphin lives in objects. But in Tales of Berseria we learn that usually, each Seraphin is linked to one object really important for them. So it's more like a symbol.

Edit :

I'll just add the link that  Namikaze Sheena mentioned in a comment for those searching more information about Seraphins and not only about Leilah or their link with objects.