The Realized Utility Services/Technical Yondard (Rusty for short) is technically not a Yondard, per se. But Mertens wanted to make the acronym work.

Mertens herself was a Yondard formed around robotic parts, forming what she is now, within a ruined base that humans once occupied. Elsewhere in the base, due to damage from an attack, Mass, the liquid that Yondards come from, got into the server room. For a while, this did little to nothing, and Mertens had, at the time, no reason to check there. But this would eventually cause a glowing fox like creature to appear.

Claiming to have come about due to Mertens lonliness, he is her only company until she meets the others. Under other circumstances, Mertens would question this, but knowing how she was born, she would decide to not press the matter.

Technically, Rusty is an Autyon, a subset of Yondards typically built around larger objects, but like Yolonda is fully self aware. The 'fox' seen is not his actual body, rather it is the projection it uses, with it's 'mind' forming in the server room. And, unlike most other Yondards, Mertens and Rusty are well aware of humans, however it is knowledge that would not amount to much.