Here was the definition of a side kick that me and three collaborators used when creating this list:
A side-kick is a mental, physical, moral, or emotional foil to a hero which increases the depth of the main hero.
Example: Mr. Spock being emotionless makes Captain Kirk's decisions and emotions seem even more extreme. Mr. Spock is a great example of an emotional foil to Kirk (but, like most on this list actually foil in all three categories).
Example 2: Robin saying, "Holy Smokes, Batman! What about...[insert analysis of villain's plot]" allows Batman to say: "Right you are, friend. But [insert deeper and more profound answer to villain's plot]." So Robin is a mental foil to Batman.
Example 3: Yogi Bear, "Hey-a Boo Boo! It looks like the pic-a-nick is out there!" Boo Boo: "Oh, Yogi, I don't know if that's a good idea..."
What does Tails do? Fly to catch up when you inevitably outrun him through half the games. At least Luigi began having legitimate thoughts in more recent games (and even his own game series).
-wtk