Carab Culln
Lord High Commander Carab Culln is the current Chapter Master of the Red Scorpions Chapter, he has worked his way up through every possible rank from Scout upwards, he served as a Sergeant in the Vanguard Veterans and a Terminator squad Sergeant, before being promoted to command the Red Scorpions 1st Company. When his total scrub of a commander died when naively meeting with old Huron Blackheart the Tyrant, Culln became the Chapter Master and led his Chapter onto kick Huron's ass in the Badab War at the head of the loyalist forces, taking advantage of the diverse chapters under his command. One quote which sums him up perfectly is as follows:
"Camouflage is the colour of fear... I have no need to hide from my foes... I have no fear of death. My colours I wear openly, they proclaim louder than any words, "I am proud to live - I am proud to die."
In short, he is a total badass of a Space Marine.
Oh and apparently he's a leviathan dreadnought now. Because reasons.
On the Tabletop
Being the Chapter Master of the Red Scorpions, the super-special OC chapter of Forge World (aside the Minotaurs), Carab has three different models to represent his progression to Chapter Master: Veteran Sergeant Carab Culln (IA 5-7, when he helped hold the line at Vraks), Commander Carab Culln (IA 4, where he was captain of the First Company during Inquisitor Lok's failed stand at Beta Anphelion II), and Lord High Commander Carab Culln (IA 9-10, as Chapter Master during his battle with Huron), with rumors of the tough old bastard being shoved into a Leviathan Dreadnought in the upcoming Imperial Armor. The only other character that even gets an alternate persona is Erasmus Tycho, and that was when he joined the Death Company (There's also IA 12's Bran Redmaw, who turns into a savage monster, but that's an in-game transformation).
Carab Culln comes at a total of 215 points, obviously takes Chapter Tactics: Red Scorpions, as well as Eternal Warrior, Orbital Bombardment, Master of Battle and Warlord. His warlord trait, Proud to Live, Proud to Die, Hard to Kill, means that basically all Red Scorpion units within 12" get a bonus to their combat score when working out who won, an even higher bonus if Culln was in a challenge, however, this rule also means that he cannot be protected by the 'Look Out, Sir' special rule. Master of Battle allows all your Red Scorpions units to use his leadership of 10 for all morale and pinning tests. He has a special melee weapon called the Blade of the Scorpion, as well as a master-crafted storm bolter, terminator armour, and iron halo and a teleport homer. His blade can either act like a regular master-crafted power sword at AP 3 or halve his attacks for a smash attack, which doubles his strength, makes him AP 2 and allows him to re-roll armour penetration tests.