Competitor might be too generous of a description, because everything about Hey is lifted and cloned from Yo, right down to the minimalist design.
Hey is still pre-beta, but it does promise to (slightly) differentiate itself by allowing you to choose how many Y's to include in your cryptic "Hey" message. Apparently, a simple "Heyy" means far more than a boring ol' "Hey."
Championing the tagline "When Yo isn't enough," Hey plans to allow users to adjust the number of "Y's" in their message by tapping their phone.
For those who think they'll still be interested when Hey enters its beta, you can sign up to try out the app here.