Vilkas

Vilkas is a Nord resident of Whiterun, Skyrim. He is a member of The Companions and is also a member of The Circle, the secret order within The Companions. Vilkas serves as the player's shield-brother in several of The Companions quests. As with all Companions, he is usually found within Jorrvaskr, located within the Wind District of Whiterun.

After The Companions questline is completed, the player can go to Vilkas to ask for more work. He will eventually give the quest Purity. He is also eligible for marriage after completion of The Companions questline.

Quests

 * Take Up Arms: Prove you are worthy to join the Companions.
 * Purity of Revenge: Retrieve the Fragments of Wuuthrad and wipe out the Silver Hand.
 * Glory of the Dead: Cure Kodlak's spirit and prove yourself as a true Companion.
 * Purity: Help a member of The Circle give up their beast blood.
 * Escaped Criminal: Track down and kill an escaped prisoner.

Personality

 * Vilkas is said to have the smarts of Ysgramor, while his brother has his strength.
 * At first, Vilkas seems to think you aren't worthy of being a Companion but, he warms up to you after you complete Glory of the Dead.
 * Farkas generally listens to Vilkas' advice. For example, he mentions that Kodlak has Vilkas and his own support in cleansing the Companions of the beast blood.
 * After Purity of Revenge, Vilkas seems guilty for having wiped out the Silver Hand. He thinks he was fueled with rage and regrets it deeply.

Trivia

 * If Vilkas is your follower, you can level up Two-handed for free. Ask him to level you up, then check his inventory and take the money back.
 * Vilkas was brought to the Companions at a very young age and was raised within the order along with his twin brother Farkas.
 * Vilkas is able to become a Blade.
 * Once married, Vilkas can be asked to move to the player's house of choice and will open up a shop.
 * Vilkas provides Master Training:Two handed training.
 * Vilkas, like all followers, can die if injured while recovering.
 * Vilkas may be randomly encountered out in the wilderness.
 * If Vilkas is your follower, he will leave you if you assault or murder an innocent civilian, just as Farkas.
 * If Vilkas is your follower and is killed, after one month passes in-game he will respawn and rejoin you. However, until he respawns, a glitch in the game will happen, keeping you from having any other followers.
 * Once he is a Blade, he will say "I'm a Blade now, I can hardly believe it."
 * The word 'Vilkas' is a Lithuanian word meaning 'wolf'.
 * The word 'Vilkas' is also a Finnish word meaning 'lively'.
 * Even though The Companions have no set leader (Kodlak could be considered de facto leader), it seems like Vilkas is second-in-command of the guild, since he is seen sitting down and having a private conversation/meal with Kodlak when you first enter Jorrvaskr, and is tasked with testing the Dragonborn's strength. He also waits outside Jorrvaskr for the players return from Dustman's Cairn, and is your shield brother during the quest Purity of Revenge.
 * When beginning dialogue with Vilkas, he may say that he claims to have killed one of every living thing in Skyrim and contemplates leaving to Morrowind.

Bugs

 * As with Farkas, there is a bug/glitch where he will draw his weapon, and talk to you constantly, as if he was a guard attempting to arrest you. If you try to wait while in the same building or fast travel, you will be unable to do so. This is most likely because he is considered a guard in-game, and will pursue you until your bounty is paid off or you serve prison time.
 * This can be fixed by simply asking him to become your follower, so long as you have one already. He will say that he cannot, and then go back to his normal routine or going to the city where you have a bounty and paying it. After that, he will no longer follow you.
 * Another option is sneaking & invisibility; with a high enough sneak skill &/or potions/enchanted items you can remain beneath his detection threshold thereby avoiding triggering his conversation script.
 * If a follower is with you during the initiation battle into The Companions and the follower attacks Vilkas, Vilkas will constantly attack you from that point on.
 * Vilkas may not offer you quests where you cure him.
 * One possible way to get out of this bug is to keep doing Farkas' radiant quests after you cure him of Lycanthropy. Make sure you do the quest where Farkas wants to kill a dragon. Vilkas' Purity quest appeared after I completed Farkas' dragon quest.
 * In the Quest Purity, he may offer to meet you at Ysgramor's Tomb, making any current companion leave. Vilkas will then remain in Whiterun and ignore the quest he gave, resulting in you being unable to have a companion.
 * If you kill Vilkas, you might find him outside of Dawnstar, frozen in the ground, screaming: "Skyrim is for the Nords!"
 * If a dragon attacks Whiterun and kills Vilkas, he may return after about one month game time.
 * Can be found in the wilds near Falkreath accompanied by Ria but they are both hostile to the player for reasons unknown.
 * During the quest Purity of Revenge, instead of fighting Vilkas may flee from battle as soon as it starts, or he may continue to follow you & fail to defend himself when attacked. (as he is a vital NPC & thereby immortal this is merely aggravting) This will happen when you take him as a follower from then on.
 * After Kodlak's funeral, when the circle meets in the underforge & agrees to honour his wish to be freed of his debt to Hircine, the Companions present cry "For Kodlak!" & charge off to Ysgramor's Tomb. Vilkas appears to suffer a recursive script error whereby he becomes stuck in a loop repeating "For Kodlak!" for the rest of the mission.

Images
Вилкас