Who's that guy?
This bearded actor with the high-pitched voice from New York is known for being really funny, having cut his teeth as man-child Charlie Kelly in the long-running US sitcom It's Always Sunny In Philadelphia.
He's married to actress Mary Elizabeth Ellis, with whom he has a two-year-old son.
Where you've seen him
As Jennifer Aniston's sexually harassed dental assistant in Horrible Bosses (2011). He also had a small part in 2010's Going The Distance.
Besides hamming it up on the big and small screens, Day, 37, is also a talented writer and producer.
A testament to Day's popularity was when he received the loudest applause - over Pacific Rim director Guillermo Del Toro - at last year's Comic-Con Pacific Rim press event.
Day adds his signature brand of manic comic relief in Pacific Rim as Dr Newton Geiszler, a scientist who's obsessed with the giant monsters - or kaiju - that are terrorising Earth.
He helps the pilots of giant mecha called Jaegers come up with a plan to defeat the kaiju once and for all.
Day is also prepping to start work on Horrible Bosses 2, which will reunite him with original castmates Jason Bateman and Jason Sudeikis.
On being handpicked by Del Toro
"I got a call from Guillermo. Apparently he's a huge Sunny fan. There was an episode called Charlie Kelly: King Of The Rats where I was bashing rats, and I was like haunted from murdering rats, and he said to himself, 'That's my guy.'"
On his Pacific Rim character
"I do think that Newton's a little bit based on Guillermo and his love of these monsters. There's some man-child in Guillermo with his boyish-like enthusiasm about everything."
On his role model
"Bill Murray. He's been funny. He's been quirky and serious. I really do enjoy comedic roles, and then if America permits me, I'll do some more serious roles after that."