Refering to the link Tuor of Gondolin suggested
I quote :-
Isildur settled on the east bank of the Anduin and established the city of Minas Ithil (which would later be named Minas Morgul), as well as the province of Ithilien. However, in 3428 Sauron took Minas Ithil, and Isildur fled northwest to Gil-galad and his father in Arnor, leaving Anárion to rule over Gondor.
From this text I assume that Isildur though proud and valiant of the race of Numenor was a bit like Boromir proud and arrogant , it is said that he fled to Arnor leaving behind Anaron to rule Gondor when the Witch King retook Minas Ithil .
Thus I believe Isildur was valiant but was mortally afraid of The Dark Lord .
I cannot find any record of this but from his behaviour it suggests so , But his brother Anarion was like Faramir , less arrogant but more wise .
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