Genuinely, Wasian Sir Pentious - specifically, half-Chinese Sir Pentious - could legitimately be a possibility for the time period he lived in.
In the 1800s, there was a big influx of Chinese sailors to the UK, who then went on to establish their own communities. Like, there were straight up enough of us that a Chinatown sprang up in both Liverpool and London sometime by the mid 1800s.
There were Chinese who married local white British people! Mostly Chinese men - since majority of immigrants were sailors - but a Pendleton with a white father and Chinese mother still would've been possible.
I feel like Pendleton's story also lends well to a Wasian reading. Racism against Chinese people was rife in the UK (classic "they're gonna steal our jobs!" reasons, then the whole mess of the two fucking Opium Wars, etc, etc) and even if Chinese people were allowed to marry British people, such unions obviously would've been disapproved of and the couples involved would've faced discrimination and racism, let alone their mixed race kids.
Pendleton was clearly, at bare minimum, middle class, if not of an upper class background. Racism abounds across all classes, of course, but if his father was an upper class man who then married a working class Chinese woman (since Chinese people wouldn't have had much opportunity to rise higher socially on their own), or even had her as a mistress and then had an illegitimate mixed race child, that would've been incredibly scandalous for the time.
Then, growing up in white, upper class British society - visibly different to everyone, never fitting in and frequently discriminated against - it's like, yeah, no wonder Pendleton ended up a hermit. Never belonging anywhere, never able to join in with wider society, pushed aside to only watch from the fringes.
And then, witnessing one of his wealthy white clients murdering a woman adds a whole other layer of complexity to the situation. Even if his parents were married, Pendleton would still have been a mixed race outcast, on top of an eccentric hermit, and it'd be his word against someone still more privileged than him. Who would've believed him? Alternatively, being an illegitimate mixed race bastard would've been even more damning for him if he spoke up.