What stage/level is manner categorized in Islam?
In the jahiliyah (period), Arabs are known for series of good and bad habits which the bad ones were erased and changed after the arrival of Islam with the rise of the Prophet as the last of all messengers of Allah and the good ones was improved and enhance among the Muslims.
The Prophet Muhammad (saw) was well-known for his good habit (manner) among the people of Makkah. Talking about his truthfulness, trustworthiness, integrity, patience, fair play, kindness, and so on before he was rose as the Messenger of Allah. It's his good good habits which made him famous and nicknamed "Al-Ameen" (the trustworthy,).
His good manners in a way or the other brought respect to his family "Quraysh", makes it easy for him to disseminate the revelation he was ordered by Allah to deliver to mankind. Of not for his character, Khadija won't have asked for his hands in business and marriage which all what she spent towards the growth, development and success of the Prophet Muhammad (Saw) would have been just a mere dream. In other way round, Abubakar too won't have allied with him in the message dissemination but because of him been well mannered.