Can't the same thing be said of characters like Hero and Link? Or Banjo and Duck Hunt? Surface level equivalence doesn't equate solely to a character's identity
No. Hero and Link are characters who do different things, from different genres, who's games play totally differently, who have totally different artstyles. Hero engages in turn based combat and uses an assortment of different spells. Link engages in real time combat and uses different items.
Banjo and Duck Hunt are again, characters from totally different genres, with totally different proportions, depicted in totally different art styles, from different eras of gaming. Hell, they aren't even the same species.
The equivalencies between Terry and Ken are closer to those between Marth and Ike than they are Link and Hero, or Banjo and Duck Hunt.
And twelve anime boys with swords and capes...
Too many. And yet, that is a reductive similarity compared to the one between Ken and Terry. Marth and Hero are both anime sword boys with capes. Yet they function totally differently in their games, they are drawn in totally different art styles, they come from different genres, and they use different techniques.