In addition to being related to and a friend and companion of the prophet (pbuh) and his supporter in all phases of his life and in different scenes of defending the religion and spreading the word of the prophet, and also, in addition to being the prophet’s son in law and having the honor of being raised by the prophet (pbuh) himself, he himself also bore some outstanding characteristics that only the prophet possessed and no other had any way to.
The depth of the imam’s knowledge regarding Allah and the hereafter and prophethood and in general, his knowledge of the unseen, his actions and sincerity in different aspects of his life, his unprecedented courage, his piety and asceticism, his sacrifice and selflessness, his patience and persistence, simplicity and generosity, his existential perfection and his encompassing all human qualities that only the elite can gain, and possessing all the necessary qualities of the apostles of God; these are all characteristics of Imam Ali (as). Some other traits belonging to him are his high rank in knowledge and his infallibility from error and sin and possessing the position of imamate.
All of these traits and qualities mentioned are his merits and he deserves them and has nothing to do with his being a relative of the prophet, because the prophet had many relatives, yet none of them ever reached his rank. Of course, let us not forget that such a thing is totally natural for a person who has been raised by the prophet and thus, he owes all of it to the prophet.
Awareness of and introducing all of the existential dimensions of such a great and high personality as Imam Ali (as) is very difficult for normal people. The least is that he can't be described in a few lines without any injustice being done. What we can do here is at least mention some of the things scholars and thinkers and writers have said about him and to point to some of his great qualities.
In addition to being the prophet’s friend and relative, companion and supporter both before his prophethood and the 23 years after it, and actively supporting Islam with his life at many times throughout this time, he himself had outstanding characteristics and a was a unique personality that no one else could ever become or reach, other than the prophet himself.
From all of the research and thorough analyses that have been done from the advent of Islam by his contemporaries and those after him, by Muslims and non-Muslims on the personality of Imam Ali (as), this reality has become clear that he is at the same level as the Ulul-Azm prophets such as Prophet Nuh, Prophet Ibrahim, Prophet Musa, Prophet Isa and Prophet Muhammad (of course minus their prophethood; he wasn’t a prophet like them).
Imam Ali (as) is a person whom all thinkers and personalities of the world, both Muslim and non-Muslim, have praised and to his greatness have given in. Shibli Shumayyil, an Arab atheist, says about Imam Ali (as): “The leader, Ali bin Abi Taleb, the greatest of the great, is the only exemplar that neither the East nor the West, has never seen the likes of, neither in the past nor the present.
Jubran Khalil Jubran, one of the greatest Christian thinkers and writers of the Arab world, says: I believe that the son of AbuTaleb was the first Arab who was able to connect with the ‘Ruh Kolli” (The General Soul). He is the first Arab to echo the tune of the Ruh Kolli in the ears of those who had heard it before…, he left this world not being able to complete his mission and get his message across to the world, he closed his eyes on this world, like prophets who would be chosen in societies that couldn’t bear their presence and were sent to people who didn’t deserve them and didn’t know their worth and came at a time that wasn’t actually the time to come.”
Petrashevsky, the renowned Russian Marxist researcher has also written about the existential dimensions of the imam: Ali was committed to religion fervently and passionately, he was trustworthy and honest, very sensitive about ethical issues and abstinent from seeking wealth and was never fond of money and wealth. Without a doubt, he was both valiant and a poet, and all of the necessary characteristics of an apostle of God were gathered in him.”
Shahid Motahhari says about the personality of Imam Ali (as):
Before being a just imam for others, and before behaving with others justly, he himself was a moderate and balanced person. He had gathered all human virtues. He had deep insight and thought, and precise and full emotions, he had both perfection in body and spirit, at night, he would cut off from all other than Allah in worship, and during the day, he would be active in the middle of society…, he was both a mystic and a wise sage, and also a leader of society, he was an ascetic and soldier together, he was a judge and a worker, a speaker and writer, all in all, he was a complete human with all of its beauties.”
Imam Ali (as), through his extraordinary talent, divine aid, self control, and the struggles and efforts of the prophet (pbuh), has attained all knowledge and as Ibn Abi Alhadid says, is the headspring of all knowledge and sciences, and the knowledge of the experts of all fields eventually end in him. Ibn Abbas says: The prophet of Allah (swt) said: “When I was prepared to intimately speak (munajat) with my Lord, He spoke with me and intimately spoke with me, I taught Ali everything I learned from Allah (swt), thus, Ali is the door to my knowledge. Therefore, it can be said that Imam Ali (as) even acquired the knowledge of prophets from the prophet of Islam and was a chest of knowledge.
One of the greatest qualities his excellency possessed was the rank of ‘infallibility’. He was infallible from all sin, error and mistake, a rank he deserved through his endless struggles and efforts and battling with his desires that was given to him out of Allah’s grace. The crown jewel of all of his virtues is the rank of imamate; because of all of the high qualities he possessed, he was eventually appointed as the imam after the prophet (pbuh) and his successor. These are all a tip of the iceberg of his personality that have been mentioned about him by thinkers and researchers, to the extent of their limited knowledge.
Ali (as) bore all the qualities and human virtues possible at their highest; courage, asceticism, altruism, patience and forgiveness, generosity and giving; all the qualities that the enlightening religion of Islam sees as human and Islamic values were gathered in him. Most of these virtues were by virtue of the prophet’s training him, as the imam himself discloses in his sermon of Qasi’ah his situation with the prophet. Of course, these qualities were also the effect of the traits he had in himself, but at the same time, he was with the prophet from the beginning, was raised in the home of revelation and was personally trained by the prophet himself.
For further information, see:
The different states of the complete individual, Question 351 (website: 349).
 See: Ardebili, Seyed Abdul-Ghani, Taqrirate Falsafeye Imam Khomeini, vol. 3, pg. 352.
 George Gordaq, Sawtul-Adalah al-Insaniyyah, pg. 19 (quoted by the translation and commentary of the Nahjul-Balaghah, Jafari, Muhammad Taqi, vol. 1, pg. 171).
 Abd al-Maqsud, Abd al-Fattah, Al-Imam Ali (as), vol. 1, introduction, (quoted by ibid, pg. 217).
 Petrashevsky, Islam dar Iran, pp. 49-50, (ibid, pg. 168).
 Motahhari, Morteza, Majmu’eye Aasaar, vol. 16, pp.g 17-18.
 Qanduzi, Suleyman, Yanabi’ al-Mawaddah, pg. 79.
 Nahjul-Balaghah, sermon 192, “قد علمتم موضعی من رسول الله (ص) بالقرابة القریبة و المنزلة الخصیصة” (Certainly, you know my position of close kinship and special relationship with the Prophet of Allah - peace and blessing of Allah be upon him and his descendants. When I was only a child he took charge of me. He used to press me to his chest and lay me beside him in his bed, bring his body close to mine and make me smell his smell…).