Merrick is an Imperial Guard Sergeant of the 85th Vendoland, and later the 8th Cadian regiment, stationed in Subsector Aurelia during the timeline of the Dawn of War 2 series. He appears to be a Stormtrooper/Grenadier sergeant, given how he appeared in the last two games with stormtrooper detachments rather than regular Imperial Guardsmen.
Why Merrick is badass
Merrick is voiced by Scott McNeil (mighty is he), which automatically makes him awesome, but no, it does not stop there, he's a prime example of what balls-of-CERAMITE Guardsmen are all about. Merrick is a total badass because he is virtually unkillable, surviving numerous battles during the Aurelian crusades. During the Tyranid invasion, he fought at Angel Gate, facing down Tyranids, Orks, and even Eldar without taking a scratch, before joining suicide mission to poison the Hive Mind. After that, he and his men withstood the Black Legion's incursion in the sector, followed by another decade of non-stop combat with Chaos, Tyranid, Ork, and Eldar forces, leading to the events of Retribution.
In Retribution, he gets killed in the first Tyranid mission by the Hive Lord, but we all know that the Tyranid ending can't possibly be canon (due to it leading to the consumption of the entire Aurelian Sector and giving birth to a massive new hive fleet), so it's no loss. More likely the first Tyranid mission happens just before the Imperial Guard mission starts. So the Hive Lord didn't finish him off, he just left him there to be picked up by Castor, and together they get payback on the Hive Lord. The mere fact he survives getting 2 bone scythes shoved through him only getting knocked out just adds to his undying awesomeness.
He is also a snarky bugger, muttering under his breath about how much he'd like to shoot his superiors. Merrick is altogether frank with his commanders, particularly to Inquisitor Adrastia, but Castor is not immune to his criticism either. One of the most badass things he does is call out Castor on his shit, beat the man to the ground, and stand defiant while LORD Commissar Bernn is HOLDING A GUN TO HIS HEAD! At the end of the IG campaign, he gives a backhanded compliment to Castor, saying he would've shot his boss for incompetence if there was any, which Castor seems to hear - and has no problem with such a guy under his command. Gaunt is impressed.
Merrick, despite seeing more grimdark and combat than most Guardsmen would see in their short services, cares a lot for his men. He cries manly tears after Typhon is subjected to Exterminatus, where thousands of Imperial Guardsmen were obliterated by the Inquisitorial Fleet. This led to him to giving Castor a shot to the gut while going emotionally awesome on him.
Merrick is also incredibly strong, being able to carry an entire heavy weapons setup on his back at a full run (though he does grumble about it). It is theorized that his own balls-of-CERAMITE generate their own gravity (akin to some Spess Elves weaponry, but powered by pure awesome), ergo, allowing him to carry autocannons and heavy bolters on his own with him only putting as much effort as lifting a lasgun.
Merrick may also be a latent Psyker--both he and Adrastia can hear Kyras talking inside their minds on Typhon, and her comments to him at the end of the campaign present more questions than they answer...
Level him up to the point where he has no setup time for heavy weapons, and watch him tear apart whole units.
If you give him the Autocannon with the ability to call down random artillery barrages and stun infantry and vehicles, sit back and watch him solo missions.
Machine Spirits don't seem to like his jury rigged repairs however, but given that the Machine Spirits are fickle cunts who throw bitchfits because you turned on the vox too loud or you kicked open your APC's door because it's jammed, it's not that surprising.
Sadly, he's still the second-weakest hero on his team, because the guard army includes both Castor and Commissar Lord Bernn. In his defense however this is like being third most intelligent guy at a table that has both Einstein and Steven Hawking sitting at it and he has the highest damage output of any IG campaign hero.