Crystals are used to Enhance Squadmates. They are used in the
"Enhance" section of the Fatal Fusion menu. Only 10 Crystals can be
used at a time and only 10 Crystals per stat may be used on any single
Squadmate. Each Crystal awards 50 points to said stat.
During Evolution, Crystals can be used on Source Squadmates to
boost its stats before evolution. This technique is handy for creating a
"Perfect Evolved Rank 4" with 7 Rank 1s. A percentage (5 or 10%) of
the stats from the Source will carry over to the Descendant, including
Crystal boosts. However, the Crystal Usage count for that Source does
not carry over to the Descendant.
The crystal usage count for a Target does carry over to the
Descendant along with 100% of the boost from using the Crystal. If the
Target has 10 AGI crystals used on it, the Descendant will show 500 more
AGI and 10 AGI Crystals used.

HP Crystals are used to boost the Squadmate's Hitpoints by up to 500 points.

ATK Crystals are used to boost the Squadmate's ATK stat by up to 500 points.

DEF Crystals are used to boost the Squadmate's DEF stat by up to 500 points.

WIS Crystals are used to boost the Squadmate's WIS stat by up to 500 points.

AGI Crystals are used to boost the Squadmate's AGI stat by up to 500 points.