Hadhrat Abu Bakr was the first caliph of Muslims. His name was Abdullah ibn abi qahafa. He was the first adult man to embrace Islam. After he embraced Islam, many people embraced Islam as a result of his conversion since he was a known personality of Makkah and people knew him for his good character. According to a Shia author Ali Asghar, “Most of the early converts to Islam were ‘poor and weak.’ But there were a few rich Muslims also like Hudhayfa bin Utba and Arqam bin Abil-Arqam. And all those men whom Abu Bakr brought into Islam – Uthman, Talha, Zubayr, Abdur Rahman ibn Auf, Saad ibn Abi Waqqas and Abu Obaidah ibn al-Jarrah – were also rich and powerful. They were members of the various clans of the Quraysh.” Similarly he also freed many Muslim slaves from their unbeliever and cruel masters, amongst them was Bilal as well. The Shia author Ali Asghar says, “In Makkah, he freed many slaves”.
The daughter of Hadhrat Abu Bakr i.e. Hadhrat Aishah became the third and youngest wife of the Prophet (s). The marriage occurred in Makkah but she remained with her parents for three years till the Muslims migrated to Madinah. It was probably 620 AD when she was married to the Prophet (s) and one year after the Makkan boycott of the tribe of the Prophet (S) ended.
Hence we can say that Hadhrat Abu Bakr had a vital role in the earliest days of Islam. He not only embraced Islam, but emancipated slaves who had embraced Islam and brought ‘rich and powerful’ people into the shade of Islam. And the Prophet (s) also married his daughter during those early days of Islam which shows that the relationship was very close.
He had also the honor of being the companion of Prophet Muhammad (s) when he migrated from Makkah to Madinah. He is also mentioned in the Holy Quran as He remained in cave for three days with the Prophet (s) during which his children would bring food for them , and inform them of the plannings of the polytheists of Makkah.
This event is also mentioned in the Holy Quran wherein Allah says, “If you do not aid the Prophet - Allah has already aided him when those who disbelieved had driven him out [of Makkah] as one of two, when they were in the cave and he said to his companion, "Do not grieve; indeed Allah is with us.”
The Shia author Jafar Subhani tries to cast a doubt on the virtue of Hadhrat Abu Bakr for his companionship of the Prophet (S) at this event and says, “It is an established fact that the Prophet spent the night of migration, along with Abu Bakr in the cave of Thaur, which is situated in the south of Makkah (a spot opposite Madina). It is not, however, clear as to how this companionship took place and this point is perfectly ambiguous in history. Some believe that this companionship was a mere chance and the Prophet having seen Abu Bakr on the way took him along with himself. Others say that the Prophet went to Abu Bakr's house that night and at midnight both of them left the house for the cave of Thaur. Still others say that Abu Bakr came in quest of the Prophet and Ali guided him to his hiding place.” 
The story of Saraqah bin Malik is well known to us. He pursued the Prophet (S) when the people of Makkah announced to give a huge prize to anyone who captures or kills the Prophet (S). When Saraqah saw the Prophet (S), his horse threw him on the ground. Unable to move forward, Saraqah realized that a divine force was protecting the Prophet (S) so he rendered apology and turned back. If Hadhrat Abu Bakr had any ill intentions, Allah would also have stopped him from reaching the Prophet (S). And how could Hadhrat Ali tell Abu Bakr about the whereabouts of the Prophet (S) when he himself was unaware and when he didn’t tell the polytheists about it. If he had not told the polytheists but he preferred to tell Abu Bakr, it shows that he had trust in Hadhrat Abu Bakr. And it could not have happened that Hadhrat Abu Bakr met him by chance on his way, for the Prophet (S) started his journey by dawn and he selected that time so that no one could have seen him. Do the Shia want to make us believe that the Prophet (S) made a scheme which was not successful in that important time when he was divinely guided and protected? And when we take these facts into account that the daughter of Abu Bakr was married to the Prophet (S) and his son and slave would come to provide food and water for them, we understand that this companionship was not by chance but by choice. The Shias always try to cast doubts on the merits of the companions in such events but in the end, they end up making unrealistic assumptions which have no solid evidences.
 Shia reference: ‘A Restatement of the History of Islam and Muslims’ by Ali Asghar Razwi, p. 47
 Shia reference: ‘Slavery from Islamic and Christian perspectives’ by Sayyad Zameer Akhtar Rizwi, p. 27
 Shia reference: ‘A Restatement of the History of Islam and Muslims’ by Ali Asghar Razwi, p. 304
 Shia reference: 'The Message' by Ayatullah Jafar Subhani, p. 227
 Quran 9:40
 Shia reference: The Message, p. 222