This is Kross, the mysterious leader of The Rouge Phantoms

Here are some facts about him

-He is Vein's elder brother

-He used to be an explorer

-He is currently deceased (He died saving a young Phroraclaw from falling through a frozen river only to fall through himself)

-His body is now possessed by an evil ice spirit.

-Kross uses deadly ice magic, enhanced with demon ice magic

-Kross formed an explorer/independent orginazation known as The Rouge Phantoms (Known members: Sabbu, Tsusaki, Sin, Kin, Vein (formerly), many others)

-Kross killed his former comrade, Lumi, who was also Shadow and Fossilsnow's foster mother

-Shadow vows revenge against Kross for killing Lumi.

-Finally, for mysterious reasons, the possessed Kross somehow still cares deeply for his younger brother, Vein.

Artwork by Haru Kitsu:
https://twitter.com/haru__kitsu?lang=en