>>8182761First things first, this dude is an Ultramarine. The various marine chapters have different heraldry akin to medieval knights.
The upside omega is the symbol for the chapter.
The red helmet, as you said, signifies the rank of a sergeant over the standard blue helm.
The red skull, like the red helm, also signifies the rank of sergeant.
The directional arrows signify battlefield role. Battleline squads get an arrow, and fast attack get the X you see here. The IV on top of it indicates squad number: 4.
Last, but not least, the trim on the shoulder guards indicates the company, of which there are ten. Like most Ultramarines you see on official artwork, this dude's from the second company who get the gold trim. But the third company gets red, the fourth gets green, fifth gets black.
