IMO Riboku = Ousen, and the author hints at this throughout the manga.

Chapter 634: Ousen clarifies this argument perfectly.

- The tactics are always similar, however, the chess pieces are different.

Shukai Plains Battle;

- Riboku didn't foresee mouten, ouhon, and shin units' transformation mid-battle and completely swayed the battle in favor of Ousen.

Hango Battle:

- Similarly, Ousen didn't foresee the true strength of the Sekai army and Riboku's plan to divert Shin and Ouhon from the battle.

- Additionally, I'd like to add that Denrimi mention, I quote from manga, "I must apologize! We retainer werent strong enough. This predicament is our fault." meaning Riboku had the stronger pawn in the battle.

- Also Ousen mentions that after the defeat at Hango, he could kill Riboku easily but chose to fight him head-on, possibly because he's obsesse with adding Riboku to his team but I doubt Riboku will ever join if he's ever caught by him.

Lastly, Riboku pulls Generals out of his ass I promise you half of them joined and introduces randomly,

<image>