The release of the new 'Kingdom Hearts 2.8' trailer today provided fans with many surprises. One of those surprises was finally getting a feel for what to expect from 'χ[chi] Back Cover', showing us characters from χ[chi] in cutscene form with spoken dialogue. One of the characters shown in the trailer was Ephemera, whose voice sounded familiar for some. A fan then asked actor Michael Johnston of Teen Wolf fame on Twitter whether he voiced the character, and he pretty much confirmed it. The tweet has unfortunately been deleted (most likely due to a NDA Johnston signed), though Johnston still retweeted a fan asking him about voicing Ephemera, which pretty much guarantees that he is the voice of Ephemera in 'Back Cover'.
Source: KHInsider & Michael Johnston's Twitter
Ephemera Voiced by Michael Johnston