With [@LHG100] stating his interest, I suppose I should also state my own as well as my ideas. Again, I've talked to Yennefir and read up on the OP to try to make something decent.

My idea for a Primarch is basically a guy who ends up in the hands of a scavenger clan who are one of many in a system rekt by metors, warp storms and a few hundred Orks. He grows up much like everyone else, scavenging and mining in the depths of space, hoping his space suit doesn't rupture or Orks decide they want some tea. However, as he grew it was clear that he was very adept at egineering and jury-rigging. Enough to the point to impress the AdMech and also getting as "Orky" with his jury rigging as (in)humanly (so no red paint speed boasts or purple stealth camo).

Eventually he would "unite" the scavenger clans, push the Orks out even even create these giant "world anchors" that would connect bits of space rock and astroids into something remotely livable. Through his efforts he had created a moon-sized celestial body and would be crowned ruler of said new "planet".

Personality wise, he's an extreme pragmatist and very resourceful, however its his pragmatist side people tend to notice more as his "tactic" for leading his Legion is literally summed up as:

Step 1) Load crap into drop pod with Space Marines in them
Step 2) Drop said Drop Pods full of guns and SPs from orbit into someplace that looks important
Step 3) Let all hell break loose

Talvyrn Gallogoth (the Primarch's name), isn't stupid or whatever, he just doesn't feel the need to make complicated battle strategies when his current one works out just find for him. The legion he'd lead would be the XIX Legion (name pending) would basically be Elysian Drop Troops on steroids in a way as they heavily rely on shock value for their attacks while also digging in so they remain a nuisance after the initial shock. 

That sound good to everyone? I fear that it might be a bit too over the top or otherwise unfitting for this RP so feedback is always welcomed ^^