In keeping with its heritage, Yooka Laylee is probably going to be a very cute and whimsical video game. So it makes sense that characters from other, similarly cute and whimsical video games, should make an appearance in it. As the trailer embedded below demonstrates, Shovel Knight – he of Shovel Knight fame – is going to appear in Yooka Laylee, but in what capacity I do not know.
Thankfully, it doesn't look like we'll need to fight against Shovel Knight: he appears to be an NPC. Whether he'll be a major or minor character isn't clear, but there's no indication he's a "pre-order incentive" or anything like that. He's just there.
Yooka Laylee is expected to release in early 2017. Here's some recent gameplay footage released during Gamescom. For the character trailer, look below: