As Annex, he wears a white costume with a cowl and metal mask, generally assembled from what was already available. He had a more unique costume later that hung on him more closely and worked with his general aesthetic.
As Kirk, he kept his head shaved and had "strikingly" jet black skin.
Kirk is capable of merging with objects to warp their spatial nature and shape. This warping ability means that his body becomes intertwined with a material, for example: Annex gets transported sometimes by warping with his teammates bikes, or once Skitter's suit. Can meld with inorganic material and reshape it. This includes things like lengthening chains, while also thinning them out, or reconstructing buildings from debris.
He was able to feel the temperature of the surfaces he was interacting with. It may have actually caused heat to leave his body during the interaction with the surface. This may have meant that he could have shown up on thermal imaging when he was moving along surfaces.While he was fluid and mobile while submerged it also effected his sensorium. He was largely limited to a sense of touch with with the surfaces he was interacting with. He could feel someone touch a wall if they were grouping for a wall but might have a hard time telling if someone was clomping around on the floor while he was in the ceiling.
His recruitment allowed the Chicago wards to go full out in wrecking their environment. Allowing Annex and Cuff to fix it later.
Liked working with Shuffle given how their powers interacted.
↑ 1.01.1“I’m a breaker and shaker,” Annex told me, “Merge into nonliving material, warp space.”
“Warp it how?” I asked.
“Reshape it,” he said. He was still half-walking, half-jogging, but he stretched a white-gloved hand out four feet, touching a sign. His hand smeared against it as though it were more liquid than solid, coloring it the same white as his glove. The sign oozed back into the wall, virtually disappearing, and Annex removed his hand, slowly reeling in the extended flesh. The sign remained where it was, compressed against the wall, the surface flat.
“Okay,” I said, making a mental note. “Okay, good.”
“While in there, I’m about as tough as whatever it is I’m controlling,” he added. - Excerpt from Crushed 24.1
↑That left Annex and Golem. Golem was uncomfortable, and I couldn’t blame him. Like me, he had details he’d want to hide. His family, his background, the fact that he was in foster care. His costume, too, was a work in progress. It was a resource for him, and maximizing that resource often set him back in the appearance department. Annex, by contrast, had settled into a ‘look’. It was plain, intentionally so. The white cloak was form-fitting, with ribs to keep the fabric straight and close to his body so it was easier and quicker to absorb. - Excerpt from Scarab 25.4
↑“Hey,” Kirk greeted us, stepping out as Golem arrived. He wore a t-shirt and yoga pants, and was glistening with sweat. His head was shaved, and his skin was a striking jet black. “You guys sparring?” -  from Scarab 25.2
↑“Annex,” I said, touching my earpiece, “They’re heading for the stairwell. Do what you can, but let them keep moving forward.”
I sent bugs ahead of the group to check the way. Annex flowed up the stairs to intercept them. Some steps became slopes instead, others had the supports removed, so the stairs collapsed underfoot. Each of Topsy’s underlings fell at some point, their burdens thrown from their arms or crushed beneath them. An unlucky or clumsy few fell more than once.
“Annex,” I said. No use. He was inside the stair’s surface. An unfortunate side effect of his power was the fact that his senses were limited while he was inside an object. He was blind, deaf, and his ability to feel was limited by the material he occupied. He could sense heat as much as the object could hold heat, could sense vibrations as much as the material could receive them.
“Annex,” I tried again.
“Back off. They’re catching up to you, and Topsy’s on his way down with an avalanche of stuff.” - Excerpt from Scarab 25.3
↑ 6.06.1Well, (Tecton) went out of his way to recruit Annex. Arguably (along with Cuff) a way to clean up messes from anything Tecton knocks down or Golem puts up. - Comment by Wildbow on Crushed 24.1
↑“I kind of like those times,” Annex said. “You get to hang out with the local powerhouses, hear what they have to say, learn from them. I had a brief stay in a few other teams, but the one thing I really like about Chicago is that everyone is okay with me asking questions, and I have a lot.”
“Who’s your favorite cape to hang out with?” Jo asked.
“Shuffle. Our powers work well together, if we’re careful not to let them interfere.” - Excerpt from Scarab 25.4
“Sort of aimed for that,” Tecton said. “They took everyone willing to leave Chicago, to support other cities that lost more members, offered incentives to the rookies if they were willing to move to another city. Your-parents-can-afford-not-to-work-for-a-year kind of incentives. I drafted these guys because I thought their powers would work well together.”
“Drafted?” I asked.
Other team leaders are going for versatility, to cover every base. I say fuck that. We build around a concept, a game plan. Once I decided on that, I went out of my way to ask for Annex, even though another team had already picked him up. Made my argument, Chevalier gave the a-ok.” - Excerpt from Crushed 24.1