Gundam Plutone, from one of the 00 Side Stories. Design by Takayuki Yanase, probably one of my favorite side 00 Gundams. It's very simple in a lot of ways, it feels very much like Yanase's usually shapes being done on a more RX-78 styled Gundam, rather than a unique one, but I enjoy that. The main giveaways of it being a 00 design are the GN Vernier designs on the shoulders and side skirts, as well as all the flat wide cables on it. I really love the leg construction with the further armor pieces latched over it, same with the forearms. Obviously this is a predecessor to the Nadleeh and Virtue. The in-story explanation is that it's meant to be an mid range unit with a GN Field, but during tests it's never able to generate a stable one. This is why you end up with the Virtue's larger design with added condensers and such for helping generate the 360 field.
Me being me I of course love a silly test design that even in-universe barely works.
Bizarrely, this is one of the units that actually got a model, relatively recently as a limited P Bandai one. It's a weird case because it has no retooled engineering or anything, just a completely new mold for a model of a Gundam that isn't even the main Gundam from the side thing it's from. Makes me happy though