Yeti is a large, hairy humanoid (or possibly just a human with a condition) who is a member of Master Payne's Circus of Adventure.
His duties in the Circus are unclear, but he is a minor Spark and owns, or at least , a gravity engine -- whatever that is. Although it's not his regular role (apparently), he steps in and plays the role of Punch in what's arguably the Heterodyne show ever staged. His performance can reasonably be described as... convincing.
While Zeetha was with the circus, he was her boyfriend, but they presumably separated without drama when Zeetha decided to follow Agatha instead of going to England with the rest of the Circus. Barring unexpectedness (always a possibility in this world), that's where he is now and will be for the foreseeable future.
With the second novelization, yeti get some more characterization and development, such as his stage freight. His backstory is also fleshed out a bit more labeling him a member of as he puts it, an utopian society of sparky individuals, this is possible substantiated by his accent, which has similarities to immigrants in the Chinese quarter of Istanbul.