Ketill Kolbjarnarson was raised as an orphan in Skjern he will join the hird as a hunter after Something To Prove is finished. Unlike all other party members who join the party, he and Nefja can't be sent away when you open the party management screen upon talking to Skallagrim or when you start the second act.
At the end of of his personal quest Buried Past, he learns his origins. As a result, he wants to change his surname. By picking the first option, his surname will be Lachlansson. By picking the second, it will change to mac Lachlian. Or you can choose a byname instead:
When Ketill joins the party, he has the following statistics. As such, he can become a great archer who could be decently skilled with knives or spears as well if enemies engage him in melee combat.
Ketill Kolbjarnarson - Default Stats and Equipment