These days I don't play many platformers, but there was a time when I did. And in that time, the undisputed king of the genre was Sonic the Hedgehog 2.
I love this game. It keeps the speed, the simplicity and the gorgeous graphics from the original, notches down the difficulty so that everyone can enjoy, and makes the whole thing work as a two-player co-operative experience.
One of the ways that Sonic 2 works is that it doesn't try and make the two characters equal and separate. Sonic is the same hedgehog we know from the first game, with a couple of new moves aimed at helping him retain momentum. He's powerful, he's vulnerable, and he has to tackle the levels in the way intended.
Tails, by contrast, is cheerfully broken. He is a fox who flies. He can helicopter around wherever he likes, completely oblivious to the jumping sequences and the avoidance of enemies. The only catch is that the screen is locked on Sonic, so if Tails gets too far ahead or behind, the player will lose control and Tails will magically come flying back to wherever Sonic is.
Sonic 2 is a game built for its demographic. When it imagines two players, it does not see two trash-talking 15-year-olds exchanging yo momma jokes over XBox Live. It sees parents playing with children and older siblings playing with younger siblings. It envisages a skilled player, and their friend.
Only one player is challenged; only one player is called on to perform. The other player merely has to join in; they can be useful, but never a liability. They can interact without detracting from the fun. It is a roadmap for bringing together the hardcore and the casual and for facilitating intergenerational play that will create lasting memories for the participants.
This is a lesson that Sega had learned in 1992 which most developers (including Sega) have forgotten today. It is perhaps ironic that Nintendo, once Sega's chief competitor, has wholeheartedly embraced this strategy while Sega struggles to find even a single audience, let alone two to bring together.