Serin

February 28, 2021

A knight of the [Tasnican] Republic who sought the Mana Sword 15 years ago. He was Jema’s best friend and [good] rival. He failed to assassinate the Emperor in the war 15 years ago [or did he?] and died in the war.

ibid.

Serin is effectively Randi’s predecessor as the world’s Mana knight.  Serin falls just short of obtaining the Sword, but Randi is successful, albeit with help from Serin’s ghost.  It seems that in death, Serin is able to watch over the Sword, as he explicitly tells Randi that he’s entrusting it to him.

Serin was a Tasnican knight who had great influence in the kingdom:

During the war fifteen years ago, Serin allowed everyone to take shelter in the castle. Because of that, everyone ended up living in the castle.

He was a close friend of Jema’s, and likely lived with Randi’s mother in Tasnica.  Serin’s death affected Jema deeply.  When the Emperor started a war with Pandora and Tasnica, Serin stepped up to fight him directly.  The Mana Tree fills us in:

During the war fifteen years ago, he and the Emperor, who had obtained the power of Mavolia, traded mortal blows with one another.  [He sacrificed himself] to defeat the Emperor…

However, the Emperor is revived by Mavolia, as part of his contract with the underworld.  Serin, probably sensing this, decides to attempt to draw the Mana Sword.  Why he didn’t do this before is unknown, but it’s possible that the Emperor’s use of Mana necessitated the use of the Sword.  

As the Mana Tree explains:

Yes… it was Serin, the Knight of the Republic—and your father—who was supposed to pull out the sword, but…

He mustered the last of his strength, and made it as far as the forest near Potos Village in order to draw the Mana Sword, where his power faded…

With Serin out of the picture, the Emperor started gathering Mana with no opposition.  This led Jema to become Serin’s successor in a way, but since Jema was not a member of the Mana Tribe, he couldn’t be the one to take up the Sword.  That’s probably what he means at the beginning when he says that the Sword can only be properly used by the one who took it—members of the Mana Tribe.  Between Serin’s death and Randi taking the Sword, there were no other Mana knights.