You will need to collect character cards or shards and fuse them to unlock the special skin (Susanoo fusion). The special character cards are imbued with active or passive skills.
Passive skills can take effect during battle without the need to change skins, while for active skills, changing skins is necessary to take effect during battle. All cards give you stats to all your characters.
You can get these cards by drawing from the card wheel. You can spin it once a day for free or by using gold/coupons.
To combine, activate the cards and upgrade them, you have to go in Susanoo then click on Fusion at the top-right corner.