Martellus is a Steve Blum-voiced Blood Ravens Techmarine who first appeared in Dawn of War II. Not being a player character, his primary job in the original installment is to deploy field generators when you capture a shrine, manufactorum, or comms array, and to provide intel of limited usefulness during deployment on missions. On the whole, he had only slightly more screen time than apothecary Gordian, whom most players don't even remember. During the final battle, his Thunderhawk is shot down by Tyranids, and Martellus plays a rousing game of skullfucking Ork Lootas and avoiding random feral 'Nids for a year.
During Chaos Rising, the force commander's squad rescues him while securing an astronomic array, and Martellus proves himself useful by fixing your Termie armor, which is horribly obsolete by this point, and guiding you to the logic engine you need to ascertain which member of your squad has betrayed you to the Black Legion. If none of your squadmates are particularly corrupt, he ends up betraying you, raising the question of why he decided to help you out in the first place, which can only be explained by incompetence. While he does get some decent lines and a fine voice, his betrayal lacks the punch of all the other ones, as the player's neither responsible for it nor capable of giving two fucks about Martellus in the first place, since he does nothing in the field. He doesn't even measure up to Apothecary Galan in good voice acting, since the latter actually had more than generic threats to offer.
Unfortunately, if you do go down the Martellus-corruption path, you will quickly start giving a fuck, either in the form of rage or fear, when Martellus
drives out of the hangar in a customized Chaos Predator hides in his METAL BAWKS. He hits for a metric fuckton of damage with his gun and ramming, summons minions and artillery strikes, and has immense health under heavy armor that puts him right up there with Eliphas and Ulkair in terms of bullshit bosses. Going down that horrible, non-canonical road more or less necessitates bringing Davian Thule with a multimelta or your other game-breaking squads. When killed, Martellus explains that Azariah Kyras is a heretic and that Eliphas saved him on Typhon.
The wargear reward you get for killing him is a power sword with a 10% chance of instakilling vehicles. It is dropped after the most difficult vehicular boss of the game.
In Retribution, it's revealed that Martellus was not the traitor, and he finally becomes a playable character, serving alongside Apollo Diomedes to hunt down Eliphas. He doesn't have as much of a role as the other three heroes do, but didn't end up voiceless like Bernn, Kain, and Varius, though his unit responses are exactly the same as Cyrus but with a helmet filter. Then again, striving to compete with the hilarity of Diomedes, the brooding of Cyrus, or the sheer awesome of The Ancient is futile, so he lurks in the background. Martellus serves primarily to point out where the next teleportarium is, and also get angry at those who misuse technology, much like a good techmarine should do.
Martellus is capable of dropping a turret that can act as a nice distraction and source of suppression or rocket fire until properly targeted, or a beacon that lets you reinforce near it, or even call in a Venerable Dreadnought to fight for you. Most of his stats are average, and he's primarily useful in buffing a vehicle-based army, which poses an issue once heavy anti-tank starts coming out in the later levels, rendering most tanks and almost all walkers useless. He's useful as an extra meltagun/plasma gun on the field, and does make a decent support hero with repairs and turrets. On the whole, he's not markedly special, but does his job relatively well.