I have 5 (Link, Robin, Lucina, Pikachu, Shulk) but I'll explain why for Robin. Robin is an extremely diverse character, with magic and swords. I'm pretty sure a fully charged side A with the Levin Sword kills as low as 45%, depending on character. Arcfire makes for a nice defensive and offensive move, Elwind is an incredible recovery. Thunder-Thoron are just fun to use, but Arcthunder has the most utility. Enemies stuck in Arcfire/Arcthunder can easily be comboed off of (Arcfire into a mid air Arcfire when they get thrown upwards.)