The description of kata is ambiguous - it says: "return a list of users and their pet names, but only for users who have at least one pet whose name starts with the letter 'M'."
It doesn't say that we should exclude all pet names which doesn't start with 'M'.
According to description we should return all users and all their pet names in case if at least one pet name starts with 'M' for a giver user.

Very nice kata! From now going on I will be more attentive to how I write queries to make them more optimal!