Soul Crystals are needed for Crew member upgrades.
Types of Soul Crystals
Red Soul Crystal
Soul Crystals used for upgrading SS Rank crew members to SSS Ranks.
Orange Soul Crystal
Soul Crystals used for upgrading S Rank crew members to SS Ranks.
Getting Soul Crystals
Click on the Sennenryu Icon which is shown here.
Click on Summon Sennenryu when it shows up.
Now, click Collect Prize to collect your prize.
You also have the option to Input Memory to the Sennenryu. This will triple the current prize by chance. It costs Gold which increases the more you try.
You have ten (10) tries to summon the Sennenryu everyday. Collect your prizes everyday to earn more Soul Crystals.