The qualities of a true devotee is found in Bhagavad Gita Chapter 12.
He, who hates no creature, who is friendly and compassionate to all, who is free from attachment and egoism, balanced in pleasure and pain, and forgiving. (12.13)
Ever content, steady in meditation, self-controlled, possessed of firm conviction, with the mind and intellect dedicated to Me, he, My devotee, is dear to Me.
He by whom the world is not agitated (affected) and who cannot be agitated by the world, who is free from joy, envy, fear and anxiety – he is dear to Me.
He who is free from wants, pure, alert, unconcerned, untroubled, renouncing all undertakings (or commencements) – he who is thus devoted to Me, is dear to Me.
He who neither rejoices, nor hates, nor grieves, nor desires, renouncing good and evil, full of devotion, is dear to Me. (12.17)
He, who is the same to foe and friend, and also in honor and dishonor, who is the same in cold and heat and in pleasure and pain, who is free from attachment. (12.18)
To whom censure and praise are equal, who is silent, content with anything, homeless, steady-minded, full of devotion – that man is dear to Me.