Sergeant Lukas Bastonne
Bastonne was a Sergeant of the Cadian Shocktroopers and was as widely known as Creed in terms of popularity. Though being noble-born, the young Lukas didn't take the typical path like his peers did by simply training to become an officer, and instead started off as a typical file-and-rank Guardsman. From that point his career took pace as wherever he was stationed, the Imperium would win. This was due to him having a brilliant memory that allowed him to to memorize orders, tactics, and his men's movements perfectly. All this combined with his brilliance, meant he could near instantly assess the situation and adapt to it in order to gain the upper hand. Due to these qualities he was promoted to Sergeant.
His prodigious memory may have been a powerful and awesome asset, yet it was also a double edged sword as the poor sod remembered the faces of every single dead Guardsman that was under his command (as well as all the horrors one in the Guard can meet). Not to mention that there were rumors that Bastonne's tattoos were the names of Guardsmen who died under his command.
Because of this memory of his, he led his men to such victories like the Defense of Bloodhaven, the Destruction of the Manthral Land-Fortress, and the rescue mission on the Forge World of Agrippina.
The latter is where he was awarded the Star of Terra, the highest imaginable distinction a Guardsman can get. The Forge World was under attack by Nurglites and Bastonne and co. had to save a Imperial Governor that run one of the hive cities there (wait, Imperial hive cities on a Forge World? Doesn’t that make it a heavily populated Industrial World instead?). The Cadians under his command battled through the plague infested streets, but even with Bastonne's tactical genius it was one hell of a harrowing fight. Even after the mission was completed, Bastonne refused with the stubbornness of a Kriegian Trooper to leave behind a wounded trooper that the squad had to leave. So the sarge went back there and brought him back to safety like a certain other badass from Catachan that refuses to leave his men behind. When he returned to Cadia, news of his heroism and success spread like wildfire, and Bastonne was celebrated as a hero.
However this was propaganda as not everything went well. The trooper Bastonne saved was infected with Chaos and Bastonne had to mercy-kill the poor sod, adding another face to his memory and probably another name to be tattooed.
A normal man would be broken after the things Bastonne experienced and remembered. Not him. Not Bastonne. He would move on and do what a Guardsman has to. No one endured trauma like Bastonne.
Book l of the Warzone Octarius supplement for 9th Edition revealed that Bastonne was martyred during the fall of Cadia, and his power sword became a relic among the surviving Cadians. So I guess we know why we haven't seen him in a while.