Two significant male characters will be introduced in the season 5 finale of ABC's Once Upon a Time, but they're not who you may think they are.

Actors Sam Witwer and Hank Harris are joining Once Upon a Time in the upcoming season finale, playing characters involved with a mental institution. Early speculation suggested that they may be playing the infamous Brothers Grimm, but OUAT co-creators Edward Kitsis and Adam Horowitz have denied this directly.

"I can say that definitely it's not the Brothers Grimm," Kitsis told Entertainment Weekly in a new report.

The mystery of who these new characters are will be part of the fun, according to Kitsis.

"What's fun about those two characters is very similar to season 1, when we didn't know who Graham was, and we were guessing sometimes what character they are," Kitsis said. "The finale will work much like that, in that the question of who these two are will be part of what the story is."

"Sam and Hank play characters who play a pivotal role in the season finale," Horowitz added. "Where they're from, who they are, and what they're doing does tee up an element of what we're doing for season 6. It's more like opening a door, rather than introducing the whole of what season 6 will be. Hopefully we're giving you a little taste of what's to come."

Even the cast members are excited about the upcoming mystery being presented in the season 5 finale. Rebecca Mader, who plays Zelena on the fantasy series, spoke with TVLine about Witwer and Harris' characters:

"It's an amazing set-up for next season. It's really, really huge. I can't wait until it's out and we can talk about it!" she said.

Mader also said of the season 5 finale, "Ohhhhh! [Laughs] I would say that you can't write this stuff, but then they did. The finale is pretty bonkers...."

Once Upon a Time airs Sundays at 8 p.m. ET/PT on ABC.

Watch the promo for the next episode of Once Upon a Time here: