Fire Emblem vs Pokemon Mystery Dungeon

Connection:

Both blade-wielding warriors from iconic Nintendo games who come from a future ruined by a draconic deity (Grima and Dialga respectively), having come to the past to prevent said beast from destroying the world

Both start off as antagonistic towards the heroes and hiding their identities, but they join their side once their true nature and intentions are revealed

Both are also related to the main heroes  (Lucina is revealed to be Chrom's daughter, the prince of the Halidom of Ylisse. Whereas Grovyle revealed that he and player, who was a human, were partners in the future where they got separated and the player lost their memory and turned into a Pokemon)

Both would soon battle the dragon that ruined everything, defeating them nearly at the cost of one of their closest friends (Robin and Dusknoir respectively), who would survive as a result of their bond.