No worries, you're certainly not obligated to know everything! In the game, Takumi would find out the following if he asked around:

-Piphron revealed himself about five months ago, or rather, he was forced into revealing himself by the people of the island who embarked on an active effort to find him
-He lives in a volcanic mountain in the middle of a wasteland
-His domain was as corrupt and barren as everywhere else on the surface and many of the characters spent a great deal of effort trying to reach his mountain
-He was reluctantly convinced to agree to lend his aid
-Since then he has been (very) gradually improving and (very) reluctantly helping
-He is considered to be the oldest of the Arehtei and was supposed to have died quite a long time ago but has stubbornly refused to do so out of distrust