Ymir and his son Jotun are Paradise Falls's pair of knuckleheads. Jotun has difficulty forming basic sentences, while his father is a jovial madman, armed with a horrific-looking super sledge, like his son's. Ymir isn't fond of Frank the bartender and makes this abundantly clear by bludgeoning Frank to death. He loves drinking, fighting, and drinking some more, rounding a day off with a spot of fighting.
When you have gained access to Paradise Falls without fighting your way in, Ymir can be found when entering the 'food section' inside Paradise Falls. The first time going there, you will probably see him arguing with the bartender, which results in the bartender getting himself killed. Talking to Ymir while he is in the argument can result in Ymir just walking away after you finish the conversation. This keeps the bartender alive, however, there are no assets or drawbacks to either option. If you ask Ymir for his reasoning, he claims the bartender had been watering down drinks so as to encourage patrons to pay for more.
Killing Ymir at any time results in a Karma gain for the player. Note that this will immediately turn all slavers in Paradise Falls hostile.