Is he who saw the Prophet (peace and blessings be upon him) and believed in him after his death, or saw him but did not believe in him until after their parting and did not see him again, considered one of the Companions? And what is the most preponderant opinion on this matter from the six authentic books of Hadith?

Whoever saw the Prophet, peace and blessings be upon him, while being a disbeliever, then embraced Islam after his passing, is not considered a Companion. This is because a Companion is one who saw the Prophet, peace and blessings be upon him, while believing in him, and died upon Islam. This has been explicitly stated by Al-Bukhari, Ibn Hajar, and Al-Iraqi. This definition thus excludes anyone who apostatized and died upon apostasy, or who saw the Prophet after his death and before his burial.