You know thinking about it Riku could also be a Lovers Arcana character. His story + arc very much revolves around his relationships with other characters (particularly his family and most importantly himself).

Captain is ultimately his arcana however because I feel like his stubbornness and his like willingness to keep going is what makes him a Chariot character (as Captain is an alternative to Chariot). He's also a very devoted/emotional character and I think that plays into it too!