Thorax is a Changeling that grew up wanting friends instead of taking love like most other changelings from Chrysalis's hive. Soon after he became friends with Spike and Sapphire, he left the hive altogether.
Thorax doesn't have that much physical difference from most other Changelings, but sometime after becoming friends with Spike and Sapphire, his wings began to shine a vibrant glow, making him different from other Changelings ever since.
Thorax was born amongst much of the horde inside the nursery hive within Chrysalis's hive, being born more timid than the others. He mentioned during his talk with spike that he was apart of the Canterlot invasion, but when he saw Twilight Sparkle and her friends, he felt that he wanted to share that love they had instead of take it.
He appears alongside Pharynx not too long after Ruby and Steven Universe was captured by Chrysalis, as he was tasked in taking their place and collect more Warp Pad pieces to finish up their own Warp Pad. He specifically was disguised to replace Ruby.
At first he was doing fine, but during a talk with Pharynx he revealed that spending time with Sapphire was making him more and more unstable, all the love she was giving off making him more hungry as he was more unstable around love than other Changelings. Pharynx was sure to put him in his place, telling him to wake up or they were gonna starve.
He was then taken by Sapphire on a date, and it was during this when he was found out by Spike when Thorax saved him from falling from a tree. Thorax reveals to Spike what his situation was, and how he wanted to share love, and how much trouble he was having due to his mother AND how all of Sapphire's love was driving him crazy. Spike agrees to help him gain friendship so he wouldn't have to feed off of it anymore, even if the very mention of love makes him ravenous. However, after a whole day of being with Sapphire, he finally couldn't hold it and exposes himself. Seeing how much damage he had caused, he broke down in tears and apologizes to Sapphire for everything. However, Sniper's Father appears soon afterwards, and attacks him thinking it was him who hurt Sniper. He was only saved thanks to Spike and Sapphire's involvment, albeit after getting his leg torn off by the Tindalos.
He was later brought to the Harmony Gems later on, and he is properly introduced to them all. During their first attempt to rescue Ruby and Steven though, he reveals that the Changeling Palace is protected by a stone that soaks away magic, and is how she kept the hive safe. He agrees to help, but unfortunately is pressured into finishing the Warp Pad inside Chrysalis's chamber, faced with the idea that his kind will go extinct otherwise. In his later attempt to help the others, he was found out by Pharynx and exposed as a traitor. He managed to escape, but was recaught again when he and the others find Chrysalis in the Gem Battlefield, just to escape when the Tindalos arrive. He doesn't show up for the final battle.