Powers: Only Agnar of all the Aesir had the power to blow the enchanted Dragon Horn of Asgard. I think that about sums it up. Perhaps Agnar could also pull off other lung-related tricks like holding his breath for a long time or knocking down opponents and blowing away obstacles with his "super-breath." Agnar also carried a sword and was called "the Fierce" so I would imagine that amongst the preeminent warriors of Asgard he must have been considered pretty tough.
Bio: After the most recent attack of the Storm Giants upon golden Asgard, Odin determines the Rainbow Bridge over which the attackers came needs a guardian. Selecting three warriors, Odin summons them to his throne room to render is decision after each makes his case for why he is the most suitable guardian.
Of the three warriors gathered (Agnar, Gotron and Heimdal) Agnar the Fierce is the first called upon to speak. After Agnar tells of his powerful lungs and mighty chest, Gotron scoffs, asking of what good is that. After Heimdall orders Gotron to silence, Agnar takes up the gigantic enchanted Dragon Horn of Asgard which none but he is able to blow and winds it, nearly deafening the gods assembled. "He speaks the truth! For ages the horn has been silent - - yet now, its roar reaches to the very end of infinity!"
Odin moves on to Gotron the Agile, who tells of his speed and agility, and finally, Heimdall, who tells of his divine senses. Kneeling his ear to the ground in Odin's lush private garden, Heimdall claims to hear the tiniest of plants growing in the Hidden Hills. Agnar accuses Heimdall of lying (even though Heimdall took his part when Gotron interrupted his demonstration) and threatens him with his sword. Odin order them all to stand fast and then commands his gardener to discover the truth of Heimdall's claim. The gardener returns with the tiniest of shoots to have only just sprouted in the barren wastes of the Hidden Hills, proving the truth of Heimdall's keen ears.
When Heimdall's eyes peer across time as well as space to detect an approaching war party of Storm Giants, Odin sends out a group of his mightiest warriors to capture them and their leader, the King of the Storm Giants. Agnar might or might not have been a member of this group of warriors.