To quote an earlier thread (2013) [webmasterworld.com]:

I thought this was a brand-new spider. Turns out it's so old I've never met it before

Punch line: The above was written by me. Well, I've had six years to forget. Current info:

UA for robots.txt: YisouSpider
UA for other requests: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/69.0.3497.81 YisouSpider/5.0 Safari/537.36
IP: various in 42.120.160-161, 42.156.136-139 and .254; 106.11.152-159
(all seem to belong to Alibaba Advertising)
first seen: at the dawn of time, apparently (see above), but more frequent in 2019, and significantly more active from September 2019
robots.txt: rarely requested
requests: mainly interior pages from one directory
unusual behavior: each blocked request (i.e. all of them except of course robots.txt) is followed by a request for errorstyles.css, a document normally only requested by humans