Writing Quality: Trash-tier. I'm not even going to bother explaining this, it should be obvious to any English native who graduated High School. One star.
Stability of Updates: Meh, pretty frequent. There haven't been any large gaps. 4 stars.
Story Development: Typical wish fulfillment BS. It's not really original or anything, some loser gets a massive power-up out of nowhere and proceeds to act nothing like how he should given his background. The story is forced and railroaded without any real attempt at making the plot progress in a smooth manner. 2 stars, for a somewhat original idea.
Character Design: The OC characters are underdeveloped, and what little we have to work with regarding their personalities fail to show in their actions. Loser MC got treated like trash and beaten to death because he looked a little weird, and is now completely OK with his new appearance, no hesitation over not being human, no paranoia over how people will treat him now, when they had already KILLED him for being different earlier. Top it all off, and Seraph, his system guardian angel thing, is one-dimensional. You'd think she'd at least have her own goals or opinions considering she's an actual being and not an AI. One star.
World Background: This fic suffers from the same common problems that are present in most fanfiction. The author comes up with "supah cool" ideas for powers, backgrounds of OCs, etc, but he makes no effort to try and explain how it would work in-universe with the pre-existing system of power, laws, and how **** works in general. I can understand some powers being considered outside context and interacting strangely with a world they weren't originally from, but at least give some kind of explanation on how this stuff worked other than, "And then his devil bringer became a Longinus!"
It doesn't matter that the Longinus are just a category of sacred gears. It doesn't matter that sacred gears aren't some naturally occurring thing in the DXD-verse, and that they were made by the God of the Bible. It doesn't matter that the only way for his devil bringer to become a sacred gear, and further, a Longinus is if God made the gear, and it was then publicly known and recognized for the power it could grant. Nope, skip any and all explanation, it's a Longinus, plain and ******.
It's the kind of lazy writing I'd expect from a 12 year old kid. 2 stars for at least having maybe read the DXD wiki to get a vague enough idea on how things work there.
Summary: Meh, if you aren't a native English speaker and are looking for something to kill time while on the toilet, here you go.