The titular Monster Collection Knights are the two lead characters, Mondo and Rokuna. Their names are written with the same kanji that make up the word Rokumon : 六 (6) and 門 (mon/gate).
They received their title from Ichirobei, but unbeknown to the professor, their entrance to Rokumon Sekai also granted them a unique power : with the phrase Adara Kadara Buu they would be able to merge with any given monster that permitted them. Through this, they would be able to aid the monster in combat, but also suffers its pain. Of such they were informed by the Fortune Telling Lady, who also informed Rokuna of her newly acquired sixth sense. All this was part of a convoluted defense mechanism of the universe itself, since while free will was nice, sometimes things went very wrong.
Mondo and Rokuna have no knowledge of the gambit and simply act on who they are, protecting the monsters while questing for the six items that can unite their own world with that six gates universe. Both children share the same dream of seeing the worlds live in harmony and introducing their people to the beauty of the rokumon world.
They generally go about their way, defeating Count Collection one way or another. Ichirobei's vendetta with Count Collection is a little mirrored in Batchii and Mondo, while Guuko is too oblivious to ever engage in a rivalry with Rokuna.
Along the way, they meet the angels and their dragons, who eventually decide to help them along on their quest.
Below is a list of all the monsters they have merged with.
They have most often merged with water monsters, counting 10 times with 8 different monsters. Earth and Wind are about equal, while Holy is a little less common and Fire and Demon a lot less common.
| # | Monster Merge | Monster Element | Primary Antagonist |
| 01. | Teppuou | Water | Collection & Co |
| 02. | Mist Dragon | Water | Collection & Co |
| 03. | Forest Giant | Earth | Collection & Co |
| 04. | Poison Toad | Water | Collection & Co |
| 05. | Trent | Earth | Collection & Co |
| 06. | Pegasus | Wind | Collection & Co |
| 07. | Green Wind King | Earth | Collection & Co |
| 08. | / | / | Collection & Co (sorta) |
| 09. | Female Rokh | Wind | Collection & Co |
| 10. | Water Dragon | Water | Collection & Co |
| 11. | / | / | Things kinda went silly here. |
| 12. | Moth | ?? | Collection & Co / Mirror Bug |
| 13. | Sky Wyvern | Wind | Collection & Co |
| 14. | Garuda | Holy | Collection & Co |
| 15. | Storm Dragon | Wind | Collection & Co |
| 16. | / | / | Collection & Co |
| 17. | / | / | Collection & Co |
| 18. | / | / | Collection & Co |
| 19. | Hundred Battle Centaur King | Wind | Collection & Co |
| 20. | Orthros | Fire | Collection & Co / Zaha |
| 21. | White Yeti | Wind | Cyclops |
| 22. | Water Elemental | Water | Bronze Golem |
| 23. | Demon Faction | Holy | Collection & Co |
| 24. | Salamander | Fire | Collection & Co |
| 25. | Hanazono No Utahime | Earth | Collection & Co |
| 26. | Water Elemental | Water | Zaha |
| 27. | Emu | Wind | Collection & Co |
| 28. | Sea Giant | Water | Collection & Co |
| 29. | Lemming | Wind | Collection & Co |
| 30. | / | / | Collection & Co |
| 31. | Pearl Princess | Water | Collection & Co |
| 32. | Chameleon Dragon | Water | Collection & Co |
| 33. | Garuda | Holy | Haunted Vampire |
| 34. | Emerald Dragon | Earth | Hekatoncheires |
| 35. | ? | ? | / |
| 36. | Forest Giant | Earth | Collection & Co / Ruth |
| 37. | Valkyrie Rookie Soldiers | Holy | Collection & Co |
| 38. | / | / | Collection & Co / Pazuzu and Bahamut |
| 39. | / | / | Collection & Co / Reda / Zaha |
| 40. | Water Dragon | Water | Collection & Co / Reda |
| 41. | Simurgh | Holy | Collection & Co / Stone Giant |
| 42. | Emerald Dragon | Earth | Collection & Co |
| 43. | Hippotamus | Water | Collection & Co |
| 44. | Forest Giant | Earth | Grand Golem |
| 45. | Black Tango Cat | Holy | Collection & Co |
| 46. | Fire Dragon | Fire | Collection & Co |
| 47. | Fire Dragon | Fire | Reda |
| 48. | / | / | Reda |
| 49. | Zaha | Demon | Reda |
| 50. | Fire Dragon | Fire | Reda |
| 51. | Saint Star Dragon | Holy | Reda |
