SA-55 is a diminutive robot with a round head composed of a semispherical cap with a smaller black semisphere under it for a lower face. His lower face can separate to reveal a cover, representing his mouth, and he has two grey-framed and pupil-less eyes. He has also a ball joint for a chest and red gloves for hands. His arms and back consist each of two thin bars that are joined by a ball joint. His lower body is a small semisphere with a grey inside and a ball joint connecting it to his back, he also features a white head cap and pink eyes and mouth, while the rest of his body is pretty much all-black with a few white joints.
In Sonic Unleashed, SA-55 assisted Eggman in his plan to use the power of Dark Gaia, which had scattered around the world when Eggman awoke it, to construct Eggmanland. He oversaw this process, doing the mechanical processes involved and relaying information of the plan's progress. When Eggman deployed his Egg Cauldron to take of Sonic in the Tornado-1, SA-55 warned Eggman that it was incomplete and to handle it gently, but was ignored.
SA-55 later accompanied Eggman on his trip to Mazuri to secure a Gaia Temple. There, he laughed at Eggman when he was pegged in the face by a rock thrown by Kwod before he was shut up by the doctor. When Sonic then came to the villagers' rescue, SA-55 came with Eggman as he escaped.
Back at Eggman's base, SA-55 presented Eggman with his dinner and reported that his plans were behind schedule, which SA-55 constantly noted was the doctor's own fault while pointing out the undesired results of his hasty actions. As Eggman contemplated on how to gather Dark Gaia's pieces, SA-55 gave his suggestions, but was brushed aside when Eggman had the idea of attracting them with an energy pulse. As Eggman's plan proved to be successful, SA-55 commenced Project Dark Gaia upon Eggman's clearance.
Later on, SA-55 was in Eggman's Egg Mobile with Eggman as the doctor was defeated by Sonic in the Egg Dragoon and then knocked into the horizon by Dark Gaia appeared. Landing in a rocky region as Sonic and Chip defeated Dark Gaia and thwarted Eggman's plans, SA-55 suggested to Eggman that he could begin anew as he listed all the ways the doctor's plans had failed, making Eggman snap and try to kick him. SA-55, however, dodged the kick and ran off with the doctor chasing after him.
SA-55's personality is rather artificial, being stiff and mechanical in behavior and speech. Combined with his sassiness, he is utterly, comically tactless. Even so, he displays an open lack of respect and invective language; he laughs openly at Eggman's physical misfortune and directly insults him, with snarky comments and sarcastic remarks at the doctor's expense.
Powers and abilities
Much like Orbot, SA-55 can retract his body into a travel-sized sphere.
—SA-55 sarcastically greeting Eggman after being knocked out by Dark Gaia.
"You can simply begin your plans anew. Even if all of your efforts this last time were utterly wasted. Even if it was a complete and humiliating loss. Even the most pathetic loser in all the world will surely have his chance to--"
—SA-55 mocking Eggman's defeat before getting interrupted.
SA-55 was never referred to by name in Sonic Unleashed, but was named in the script, with the meaning being that it is leetspeak for "sass".
He was nicknamed "Ergo" by fans due to his frequent use of the word while dissecting Eggman's plans.