It's a phone app. Essentially it tracks your sleep quality and in the morning after every sleep cycle, pokemon will appear and you can feed them to become your companions.

There's no combat and minimal attention required. I just open the app before I sleep, and feed pokemon some biscuits in the morning.

It tracks using the mic, so it might be draining on the battery. Probably not the best thing for privacy either, but it's made me sleep on time since it stops me from picking up my phone on the bed.