Regardless of Gaimou, let's get a few things straight about Man'U
-Was Man'U designed to be a hurdle to Moubu strength wise?
Definitely not, Man'U being weaker than Kanmei was stated in the manga
-What is the purpose of Man'U?
Character development for Moubu
-Who was the main character of ManU's flashback?
Yes, its Moubu lol
''He was betrayed by his burden''
This literally summarizes Moubu's character arc in Chu futurely, he gets betrayed by the one man who can tame him. Not only does he switch factions, but SHK also crushes Mouten in battle. This will be a devastating blow that Moubu will deal with through Man'U
Man'U is a big bloke with hefty weight, he is in the top 10 -I have him on 7 right now-, but he is not some unstoppable force in the verse, this is a personal battle for Moubu. Houken destroys Man'U and his sob story, naturally that goes for Ouki, Renpa and Kanmei too
Gaimou is yet to be properly gauged, but if we go by the narrative of Gaimou representing the final obstacle in Wei, at least martially, then there's a decent chance he turns out stronger.