Adler's character Karofsky was struggling to come to terms with his sexuality last season.
Adler has now told E! Online that viewers will be surprised when he next appears on the show.
"Karofsky is back, and it's something I don't think you'll expect," he teased. "You'll see him at a place that I don't think anybody would really guess."
Adler continued: "It reminds me of the 'Never Been Kissed' episode, where you have this secret that you can't wait for the audience to see. And I can't wait for it to be shared and hear people's reactions. I think the audience gets more out of not knowing and having that shock element."
Glee airs on Tuesdays at 8/7c on Fox in the US and on Thursdays at 9pm on Sky1 in the UK.
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> Glee: 'I Am Unicorn' recap
Take a look at some photos from the latest episode of Glee, 'I Am Unicorn', below: