Marik Ishtar is a character that's suspected to be coming soon. There are several files found in the game showing a character selection icon, and a player profile icon. Similar to Yugi Muto, he has an alter ego called Yami Marik. His Yami version is more likely to be an unlockable character to avoid having 2 of the same character. Marik Ishtar might make an appearance through a mini event just like the Yugi Muto event.
Based on the anime, Marik Ishtar takes control of other people and use their decks to duel and does not duel himself, unless he is in his Yami Marik form. If he were to appear, he could be bringing other gravekeeper monsters like Gravekeeper's Descendant, similar to Ishizu and Odion's duel rewards.
According to the news, a new Legendary Duelist is set to appear toward the end of May. Upon opening the app once last week and twice today, I've heard Mario's voice in the title screen. So Marik is definitely coming.
Just so everyone knows. These are cards he is expected to use, NOT GIVE OUT. Everyone was afraid of tribute to the doomed and sakuretsu armour being added to the game before bakura's mini event, but we still can't get them. He may give some of these, but don't fret over the game breaking ones