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 gets some more characterization and development, such as his stage fright. His backstory is also fleshed out a bit more; he claims to be a member of a utopian society of sparky individuals hidden away in the Himalayan Mountains near Tibet. This doesn't seem to be supported by his accent, however, which sounds the same as immigrants in the Chinese quarter of Istanbul. Also, the gravity engine becomes "the gravity equations", which, when correctly contemplated, allow him to strengthen the force of gravity in a selected area; he successfully slows down the Monster Horse Beastie in this fashion.