The Phoenix's soul split into 5 Mystery Disks. That's all well and good... until you look at the disks and realize they were Lost Disks. Meaning the main characters started out dead until the Phoenix inhabited them.
Who figures out that the five Searcher Monsters might be the soul of the Phoenix? Mocchi. The youngest. And at first he doesn't even realise that it would mean giving up his own life.
A lot of ways to unlocking monster contains this. However the mind blowingly over the top unlocking requirements of Beaklon that appears as if it's an Urban Legend of Zelda actualy has this. Remember that in order to get it, the worm needs to be C Rank or less ? Other than the cup jelly, this is the requirements that seems to be out of place. No more if you realize that Beaklon is rank C in the card.