Karkaroff, Igor: The former headmaster of Durmstrang, who ran away from his Death Eater obligations and was killed for it.
Etym: Couldn't find anything on this name, but might be invented from the Russian stem karka- "to caw, to predict the worst". Another outside possibility is Finnish karkuri "deserter, runaway".
Karkus (OotP ch. 20): The Gurg until he was killed by Golgomath.
Etym: Maybe from carcass?
Etym: St. Katherine or Catherine of Alexandria was a scholarly woman who was imprisoned for converting the wife and soldiers of the emperor Maxentius. He is said to then have dispatched his philosophers to convince her by logical arguments to denounce her faith, only to find that she instead converted them. Later she was martyred, and a particular instrument of torture associated with this is known as "St. Katherine's Wheel". The etymology of this name is unknown.
Keeper: The player on a Quidditch team who attempts to keep the Quaffle from going through any of the goal hoops.
kelpie: A malicious water creature able to assume a number of shapes, but usually appearing as a horse, which takes delight in the drowning of passers-by.
Kendra Dumbledore (DH ch. 2): Etym: Feminine form of Kendrick, which comes from Old Welsh Cynwrig, which might be from worlds for "high, exalted", and "hill, summit".
Kenmare Kestrels (OotP ch. 11): Seamus Finnigan's favorite Quidditch team.
Kenneth Towler: Etym: Unknown, but the name of one of the most popular Scottish saints and three Scottish kings.
Kent: The home of the Wailing Widow.
Kettleburn (PoA ch. 5): The former Care of Magical Creatures teacher.
Etym: Means pretty much what it looks like.
Kevin (GoF ch. 7): Etym: See below.
Kevin Whitby: Etym: From Old Irish Coemgen or Caemgen "comely birth". St. Kevin is one of the patron saints of Dublin and is represented as a protector of animals.
Killing Curse: One of the Unforgivable Curses, a spell of instantaneous and terrifying death; fod; kill -9. The only person ever known to have survived it is Harry.
King's Cross: A major train station in London, and the point of origin for the Hogwarts Express. Rowling has said, though, that when she described the station, she was actually thinking of the layout of Euston.
Kingsley Shacklebolt: Etym: Possibly after Charles Kingsley, a Victorian novelist, clergyman, and teacher who was a strong supporter of social causes.
Kirke, Andrew (OotP ch. 21): One of the replacement Beaters on the Gryffindor Quidditch team.
Etym: From Middle English kirk "church".
knarl (OotP ch. 9): A hedgehog-like magical creature which can fire its quills at people. (Hedgehogs were once believed to be able to do this).
Etym: A 19th-century word which meant "hunchback" or "dwarf".
kneazle (OotP ch. 15): Something that was going to be covered in Care of Magical Creatures.
knickerbocker glory: A confection similar to a large ice cream sundae with all the attendant toppings, sauces, etc., traditionally served in a very tall glass.
Knight Bus: A bus summonable by any wizard in dire need of transportation which can take them anywhere they want to go on land.
Knights of Walpurgis: An earlier name for the Death Eaters which Rowling mentioned in an interview. Maddeningly, she did not say whether this name belonged to an organization predating Voldemort, or Voldemort came up with this name too.
Etym: From Walpurgis Night, a pagan festival absorbed into Christian tradition as the last stand of the forces of evil, represented as witches, until Halloween.
Knockturn Alley: The evil twin of Diagon Alley, location of Borgin and Burkes.
Etym: Say it out loud...
knotgrass: Polygonum aviculare, and one of the ingredients for the Polyjuice Potion. Herbal lore says an infusion of knotgrass is supposd to stunt one's growth.
Knut: 1/493 of a Galleon, 1/29 of a Sickle.
Etym: Not sure, maybe just a variation of nut, the coins being brownish and all.
Kreacher (OotP ch. 4): The loyal, long-suffering, put-upon, etc., etc. house-elf steward of the Black family's house in Grimmauld Place.
Etym: Mangled spelling of creature.
Krum, Viktor: The Seeker on the Bulgarian national Quidditch team, and the Triwizard Champion for Durmstrang. Wand: 10.25", rigid, hornbeam and dragon heartstring.
Etym: This was the name of a Bulgarian khan who died in 814, who developed the first rudiments of state organization there and was able to threaten the Byzantine Empire toward the end of his reign. Krumm, German for "crooked, bent, winding, twisted" is mentioned on other HP Web sites, but this seems an unlikely source to me.
Kwikspell: A correspondence course in basic magic that Filch apparently tried.