Shri Krishna defines a devotee in the Bhagavad Gita

It was a dawn which would be a different one from every morning. It was a day to witness the start of a fight of two sides, one which stood for power and ego, while the other stood for truth and justice. It was the start of the great war of Kurukshetra. But it was only later that the even more importance of the day was realised, when Shri Krishna fulfilled his ultimate responsibility to give the world the real meaning of life and Brahman, through his discourse of the Bhagavad Gita which he first delivered to Arjun, but which was also heard by Sanjay, and King Dhritarashtr, through which the entire world came to know. It was a day meant to change how humans understood life, and why Shri Krishna is aptly called the Jagadguru.
In the start of the war, Arjun who was to fight the great warriors like Bhisma, Drona and many more falls into a depression watching and thinking of the moment he had to kill them all. Shri Krishna notices the change and decides that it is time for Arjun to get enlightened on the duty of Karma and thus he starts while revealing Arjun the truth and real facts of the cycle of life and death and how the real cause of all is himself, and humans were his means to attain their karma. One of the important question which Arjun raised during this, was that of who is a true devotee of the Almighty and how would one recognise him, for which Shri Krishna looking fondly at Arjun defines true devotion.
Shri Krishna elucidates the qualities of a true devotee and the first one which he points out it the one who is friendly and caring with everybody. A devotee who has no ill will, free of ego and believes in ‘us’ rather than ‘me’ won his heart, mentions Shri Krishna to Arjun. A true devotee is the one who is always satisfied, self-restraint, has control over his senses and who believes in courage and determination to get on with all the challenges of life.
Shri Krishna further defines a devotee as the one who would do his duty in complete focus rather than worrying about the results of his actions, with trust in Almighty. It was a true devotee who would have the courage to treat a friend and foe in the same manner and who would be free from attachment, desire, love and hate.
Shri Krishna says to Arjun to focus his mind over his karma and let everything happen as it was Shri Krishna who was doing the cause, and Arjun‘s prime duty was to meditate upon him as a true devotee who would never stop doing his duty, understanding it perfectly well that it is the part of the plan of the Almighty.
Finally Shri Krishna Says that the faith a devotee has is always in the Almighty and it is the Almighty’s heart that the devotee calls his home and place in the Universe and it was such a person that Shri Krishan favoured as faith in Shri Krishna is faith in everything. Revealing the duty of a true devotee, thus Shri Krishna stresses on Arjun that a true devotee if he is, would fight for the cause of Dharma for which even the Almighty takes birth each and every Yuga.