How is Arjuna a devotee?

Now people may say that "He was not chanting. He was fighting. How he became a great devotee?" But Kṛṣṇa says, bhakto 'si priyo 'si me: "You are My dear friend. You are My devotee. Because you are utilizing your energy for Me." So bhakti means you utilize your energy for Kṛṣṇa. Then your life is perfect.

Śrīla Prabhupāda Lecture – Bhagavad-gītā 1.15 – July 15, 1973, London