The most appealing and interesting characters from the Fire Emblem series, spanning 17 games, in crossovers with characters from different entries from the same franchise.
Sometimes, crossing over with characters from other franchises that share a similar theme.
Example 1: Nergal from FE7 and Hilda from FE4 are both power-hungry magicians that lead an army of revenants among ordinary troops, but Nergal is a tragic victim of fate, while Hilda is a bitch outright, so they are a match
Example 2: Nyx from FE14 and Yeul from FFXIII-2 share the "wise old woman in a young teen body" trope, so they are crossed over together because they fit thematically.