DI is not character specific.
This thread will explain DI to you.
http://www.smashboards.com/showthread.php?t=221969
The thing is, is that characters have better aerials to slow their momentum than another character, or they have different aerials that are better suited when flying off the stage upward or toward the side. For example, for the characters I know best, when Peach is knocked toward the side of the stage, her best DI is hold up on Control stick -> Uair -> Fair/Toad. While for Ivysaur, When she's knocked to the side, it's control stick up -> Fair -> Dair.
In case of being sent upward, for the same characters, Peach is Nair -> Fastfall . While Ivysaur is Uair -> Fastfall.
While otherwise, as we know, GnW has the Bucket-cancel which stops his momentum completely.
As for Sheik, She has a problem with DI since her moves don't aid a particularly noticeable amount in momentum canceling. Potential momentum canceling moves for Sheik are Fair and Uair, because you have to calculate not only how fast the move is, but have fast it ends. The problem with Sheik's Dair is that it takes too long for the move to end, so you end up still drifting while she's still trying to get the move to finish, ending in a lost stock.
In my opinion, Sheik's best DI when knocked sideways is Control stick up -> Fair -> Fast fall -> Fair. It's the quickest, and ends the fastest, but it still doesn't help a whole lot, but still...