Arella von Mekkhan is the of Vanamonde von Mekkhan, and therefore the widow of Carson von Mekkhan's son, who attempting to defend Klaus Barry Heterodyne, the infant son of William Heterodyne and Lucrezia Mongfish, during the attack on Castle Heterodyne.
It's Arella who brings Carson the news that a claimant to the Heterodyne throne has appeared in Sturmhalten and may be on her way to Mechanicsburg. Having seen many pretenders enter the Castle only to be summarily , he's skeptical, but Arella has been strolling around town as usual, listening to the rumors of what happened, and she thinks this one may be ... different. She has no idea how right she is.
At any rate, she is able to convey enough of the remarkableness gathered from the rumors to galvanize old Carson to rouse himself from his comfortable retirement, at least for a little while, to wait at the city gate for her to appear. Once he encounters Agatha and company, all manner of fun starts, so although Arella isn't on stage for very long, she plays a pivotal role in the story. We then saw her again at the beginning of Volume 10 when Vanamonde returns to find Carson affected by the apparent "death" of Castle Heterodyne.
She also appears in Volume 11 when Baron Oublenmach takes her and Carson hostage.