Possible name origin
"Earnest" = English for "serious;" also used to refer to something put up as a pledge of one's faith, such as 'earnest money.' See also Ernie Rowling.
Profile by Steve Vander Ark
Though sort of a pompous fellow, Ernie's heart and loyalties are in the right place. Harry first met him in his second year, when Ernie took the lead among the Hufflepuffs in attempting to protect Muggle-born Justin Finch-Fletchley. Although at first he believed Harry to be the Heir of Slytherin, Ernie saw reason when Hermione was Petrified, and gave Harry a public apology, which was accepted (CS).
Ernie became a prefect in his fifth year, together with his close friend Hannah Abbott (OP10). At the beginning of that year, when the Daily Prophet's smear campaign was at its height and Umbridge was torturing Harry in nightly detentions, Ernie spoke up very loudly and publicly in support of Harry and Dumbledore in front of a grateful Harry (OP13). Ernie later suited action to words and joined Dumbledore's Army (OP16), even joining the group that cursed Malfoy on the Hogwarts Express at the end of the year (OP38). He later fought in the Battle of Hogwarts, leading the crowd that wanted to stay at school and fight (DH31) and helping save Harry’s life with a Patronus (Ernie’s was probably the boar) (DH32).
In the Movies
The role of Ernie Macmillan is played by Louis Doyle.