In Arabic, the word ‘adl (translated here as person of good character) means upright or straight; its opposite is crooked.
In sharee’ah terminology it means the one who fulfils the obligatory duties and does not commit major sins, or persist in committing minor sins.
What is meant by fulfilling obligatory duties is things like the five daily prayers.
And he does not commit major sins such as spreading malicious gossip and backbiting.
As well as being of good character, he must also have strong eyesight, so that his claims may be believed. If he has weak eyesight then his testimony cannot be accepted even if he is of good character, because if he gives testimony even though his eyesight is weak, then he is deluded.
The evidence for that is the fact that Allaah has made strength and trustworthiness among the reasons for offering employment to a person. In the story of Moosa and Shu’ayb, one of Shu’ayb’s two daughters said:
“O my father! Hire him! Verily, the best of men for you to hire is the strong, the trustworthy”
[al-Qasas 28:26 – interpretation of the meaning]
And the ifreet among the jinn who was commanded to bring the throne of the queen of Saba’ said:
“And verily, I am indeed strong and
trustworthy for such work”
These two characteristics are the basis of every deed, including giving testimony.
Al-Sharh al-Mumti’, 6/323. For more information see al-Mawsoo’ah al-Fiqhiyyah, 30/5, Kuwait edition.