Cortes is a monster bounty hunter hired by IMa to look into the missing Mocchi incident. For a while he believes it to be a monster stealing the Mocchis, and when you meet him at the Bugle Inn, he asks you to take a Mocchi decoy doll to the Kalaragi Rainforest and see if you can draw it out. It turns out to be Geras, and Cortes goes after him when he evades capture.
As a side note, Cortes carries a dagger that looks suspiciously like the one that Holly carries in the Monster Rancher anime series.