Though I personally don't RP, it may be important to think about how late in a character's career one can acquire one of those weapons. Most will not get one until their 40s (unless lucky enough to get SKed during the right mission to get a badge).
Aside from that, one could RP nearly any reason to have a sword with a moving eye-ball on it. It needn't be because they got it from a Rularuu-related task specifically.
Heck, one could claim that your character posesses a living magical weapon, and as it (not the player) gains power (levels) it evolves until it reaches its final form (the Rularuu-style).
Unlockable Weapons as per ParagonWiki should tell you the how and where of getting the Ruularu sword. (Has a special Ruularu list after the table-chart thingy)
As for RP reasons for having it, well, that's up to you. Although (for consideration) I have a now unused character concept about a Hero who possessed a sword that was (eventually) tainted by Ruularu magics/dimensional-powers/whatever, causing the sword to gain sentience.
But, as the plot would have it, the character found the will to control the sword and there he was, Ruularu blade in-hand.
Hope this helps! 
- El D
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I am needing some help concerning this subject.
I was wondering what a good RP/lore explanation of a character having a Ruularu weapon would be?
I'm not sure exactly how a hero or villain would get an item like this (RP-wise), so I wanted some other opinions.
Thanks for your time!
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