Slade is a minor character introduced during King's and Gale Glory's flashback. He was a member of the Empire.
Slade was a brutal member, who slaughtered many of Demon Card, despite Gale's and King's pleas not to. He was also nice to Gale Glory outside of his missions. That is until he found out, that Gale also founded Demon Card.
Slade greets Gale Glory, who wants the man to arrest Demon Card. When they arrive Slade kills every Demon Card member except King, who he imprisons, and Lucia, who turned out to have survived, who got also imprisoned afterwards. When he found out that Gale also founded Demon Card, he attempts to kill Gale until King attacks him from behind, killing him, and is revealed to be too strong to be imprisoned.