The story of Mahabharata includes many characters and each of them has it’s own influential role in the epic. Since it is a composition of Ved Vyasa, hence it centres around the supreme lord and hence, it is meant for people who are devoted to him and wish to enjoy his pastimes in various forms.
Since Krishna is never alone, he is always surrounded by his devotees and hence, while mentioning of Krishna, his devotees are always mentioned.
In Mahabharata, there are many devotees of Lord Sri Hari and each of them has a separate role as well as eternal relationship with him. There are five major relations and seven minor relations.
The five major relations are:
Vatsalya (Parental affection)
Madhurya (Conjugal love)
Arjuna is related to the lord in the relation of Sakhya i.e. he is a dear friend of the lord. The lord himself states in Bhagwad gita(4.3)
sa evayam maya te ‘dya
yogah proktah puratanah
bhakto ‘si me sakha ceti
rahasyam hy etad uttamam
That very ancient science of the relationship with the Supreme is today told by Me to you because you are My devotee as well as My friend; therefore you can understand the transcendental mystery of this science.
Here the lord states that being a friend of the lord, he is telling him all the deeper aspects of philosophy.
Arjuna was the third one in the Pandavas and was the son of King Pandu and Queen Kunti. He was born from Kunti when she called upon Lord Indra to scrutinize the mantra which Sri Durvasa Muni had granted him when she selflessly served him and his disciples.
Arjuna was tormented by the Kauravas and hence, being his friend, Sri Krishna specifically carried out the peace treaty which was in vain and hence, was on the side of his devotee in the battlefield.
In the midst of the batttlefield, he presented himself as grief stricken and unable to fight and then, became a disciple of the Lord. After that, he didn’t present any arguments and simply listened to and did what the Lord said. And he approved the lord that whatever you say, I accept it as it is.