You'll be able to square off against God of War protagonist Kratos in the PlayStation versions of Shovel Knight, developer Yacht Club Games has announced.

This confirms what was teased in the trailer above, which originally aired during the PlayStation Experience back in December. Kratos drops onto the screen and appears to be a boss you'll fight, and it turns out that is indeed the case.
However, as detailed on the PlayStation Blog, Kratos isn't someone you'll inevitably stumble across. Encountering him is described as being "very tricky," so you'll need to "keep your eye out for secrets… and possibly even double secrets" to get him to show up on the world map. Defeat him, and it sounds like you'll unlock some new abilities--Yacht Club teases that "Shovel Knight learns a trick or two from Kratos."
When you do face him, you'll be treated to a Shovel Knight-style version of the God of War theme music, which you can listen to below. The GIFs above and below should also give you an idea of what Kratos is capable of.
Shovel Knight releases on PS4, PS3, and Vita on April 21 as the final game in the 2015 Spring Fever promotion. It'll feature Cross-Buy support and be priced at $14.99, or $13.49 during its launch week for PlayStation Plus members.
The retro-style platformer is also headed to Xbox One, where it'll feature an appearance by the Battletoads.