Fobos

Fobos is one of the Four Philosophers.

Since being freed from the La-Mulana ruins and completing his duty as a Philosopher, he's been living in the Gate of Illusion trying to recreate the ruins that were destroyed and the relics that were lost when the protagonist's father, Lemeza Kosugi, solved the mysteries of La-Mulana.

Visiting him, he can give you hints throughout your adventure, so visit him often!

Ankh Jewel Repair
After defeating your 3rd Guardian, Fobos will give Hiner an Ankh Jewel, which you can buy from Hiner's shop in the Gate of Guidance for 50 Coins.
 * He does this a second time? I'll update this when I figure out exactly when that is. For now, just keep an eye on your Xelputter for Fobos texting you about it.