Arrena is the natural daughter of Lord Jorthos Harrigon and a peasant girl named Maella. Her father took her and raised her at Smithestone, intending to raise her as a blacksmith along with all of her other siblings, true-born and baseborn.
Arrena had been working in the forges of Smithestone for a number of years until her father married her to Ser Ethan Snow, the master-at-arms at Helgen in the North. This was part of an alliance with the Dragens, which also married Arrena's half-sister Larinna to Lucius Dragen, the new lord of Helgen.
She knew that Lucius was abusing Larinna for weeks after their marriage, but never attempted to stop the violence, as Lucius threatened her and Ethan if she tried.
Arrena soon gave birth to Merei, whom she and Ethan gave the name Whitesnow to in order to remove the trace of bastardry from her. Arrena was with her sister Larinna in her birthing chamber, the first time when she gave birth to Warron and a second time when she gave birth to Melina.
Arrena is made aware that Ethan Dragen, now the new lord of Helgen, is being pressured by House Dragen's vassals to divorce Arrena and marry a woman more suitable, as Arrena is still a bastard. Maester Vahaelor hears of this and fears that if this happens, House Harrigon would end their alliance with the Dragens. In a short amount of time, both Lord Ethan and Lord Rickard both agree to have Arrena legitimized.