Bhaktavatsala is none other than Lord Shiva himself, so called because he is very kind towards his devotees. He is also called Bholenath for this reason. Whoever offers prayers with at most dedication, Lord Shiva showers his grace upon them, irrespective of their race, cast, sect and gender. It is also very much evident in all the epics.
There are few devotees of Lord Shiva who have been categorized in the top level. Some of them demon king Ravana, Lord Brahma, child devotee Markandeya and the two associated of Lord Shiva Nandishwara and Nagendra. The demon King Ravana has been the most revered devotee to Lord Shiva. It is true that there is no other disciple to Shiva like Ravana.
Ravana starts praying Shiva even before entire world wakes up. He knows every Puja performed for praying Lord Shiva. Even at the time of war with Lord Rama, Ravana did perform the most critical pujas , not to win over Rama as he did not have fear of death, but to stay alive to offer prayers to Shiva. Such is his devotion towards Shiva.
Lord Brahma and Goddess Saraswati are said to be the foremost disciples of Lord Shiva. They knew that without the grace and permission of Lord Shiva, neither can Brahma create the beings, nor can Saraswati be the Goddess of Wisdom.
If Markendeya would not been a devotee to Lord Shiva, he could neither have taken birth, nor sustain the life after getting birth. In the same way, neither of Nandishwara and Nagendra could have gained such importance in the history, if they were not associated with Shiva.