* In Norse mythology: The Kraken is often depicted as a monstrous sea serpent or squid, and it's not usually associated with a specific ruler. It's generally seen as a force of nature, powerful and dangerous, but not necessarily ruled by anyone.
* In modern fiction: The Kraken has been portrayed in many different ways, and its "ruler" can vary depending on the story. Sometimes it's depicted as a powerful entity itself, with no master, while other times it might be controlled by a powerful wizard, an evil god, or even a more cunning, intelligent kraken.
* Real-world interpretation: There is no evidence of krakens existing in the real world, so the concept of a "ruler" is entirely fictional.
Ultimately, the answer depends on your interpretation of the Kraken and the fictional world it inhabits. If you're writing a story, you get to decide who (if anyone) rules the Kraken.