Trespasser, one of the most talked about kaiju on this wiki due to his mysteriousness. My theory for Trespasser is that he was sent out to quickly get rid of us the vermin, my theory is in short terms is just trying to comprehend how he took more nukes than any other of the first four kaiju, although we do not know how powerful the nukes used on him were but still three and I repeat three nukes is still a feat of strength. Trespasser could be category 3 -4 in my eyes but most people would disagree since he was the first.One thing that doesn't make sense with Trespasser is his statistical values, his stat line is pretty bad when compared to other kaiju which suggests he is of a lower category, but his size does not suggest he is a low category in fact he is taller than Leatherback and Otachi category 4s, but the serizawa scale does not just include height nor does it entirely rely on stats. We do not know his ambient radiation or his toxicity, but based on his encounter with yeye he is not that toxic but then again in the graphic novel Gatling gun rounds make him bleed, while missiles in the graphic novel make him flinch which doesn't make that much sense due to Trespasser being seen in the movie being quite tough just taking the planes shooting at him and blowing up on his hide.
To conclude we can not correctly assign Trespasser a category without a proper confirmation from Travis Beacham, Del Toro or some other reliable source.