Denys Mallister is a knight and the commander of the Shadow Tower, the westernmost of the three still operational castles of the Night's Watch. He is the uncle of Lord Jason Mallister of Seagard.
Ser Denys Mallister is a senior member of the Night's Watch. He has commanded the Shadow Tower, the western-most castle still manned by the Watch, for 20 years.
Following the death of Jeor Mormont, a choosing for the new Lord Commander of the Night's Watch will take place. Samwell Tarly tells Gilly that Denys Mallister is running against Ser Alliser Thorne. He states that Denys is a good man and thinks that he wouldn't send Gilly away, which is what will probably happen if Alliser is chosen.
Denys is present at the choosing for the 998th Lord Commander of the Night's Watch, but ultimately loses to Jon Snow. He salutes Jon to congratulate him.
The oldest ranger in the Night's Watch, Denys Mallister took the black as a boy. His adept command of Shadow Tower made him a strong candidate for Lord Commander, but he lost the election to Jon Snow.