Although his images were highly realistic, they very seldom crossed the line into hard-edge style. His subjects were often posed in interesting ways (aside from in many commissioned, official-appearing portraits). In general, I find his works interesting, pleasing and impressive thanks to his skill in making them.
On the other hand, Jagger's paintings are so reality-oriented that they often give no hint of a personal style. To put it another way, his personal style was so attuned to representation that, in most instances, it can be hard for a viewer to think "Aha! That's a Jagger." Exceptions are some dramatic paintings of women that might be said to have a "Jagger look."
Apparently not much is known about his life. For what it's worth, here is his Wikipedia entry. And for your amusement, you might try this link wherein an art scholar (I presume) tries without much success to fit Jagger's work into a 21st century ideological procrustean bed.
Below are examples of his work. For more examples of his commissioned portraits, link here.
This shows how well Jagger could nail his subjects' appearance.
According to Wikipedia, this is Jagger's most famous painting. That said, I don't consider it his best or most interesting.
He was able to land commissions from important people.
The date by the signature looks a lot like 1914, but the clothing is more suggestive of the 1917 British army. This shows that Jagger could paint freely if he chose to do so.
A nice, dramatic pose and many soft edges help us to focus on her face.
No date on this one either, but Kathleen looks a few years older than she did in the previous portrait. Much thin painting here. Perhaps Jagger was experimenting. Some of the paintings below might also be of Kathleen (note the eyebrows).
I don't notice a fan, but that's the title the Internet gives me. Regardless, this is a very interestingly composed painting that looks like it dates from the mid-1930s.
The Internet has it that it's a scarf, though to me it almost seems she's wearing what looks like a shawl, due its size. This painting is more thinly painted than most of the others shown here, revealing that Jagger could and did alter his style at times.
A dramatic expression on a face with odd features -- note the eyes and area around the mouth. The hand and cup/saucer are hard-edge, unlike the rest of the painting. Makes me wonder if these details were unfinished in spite of the artist's signature indicating completion.
Yet another of those dramatic, interesting poses. Notice that Jagger places a dark background object behind the subject's head to create a central dark zone from top to bottom, contrasting with the light gray background areas.
Here Jagger delves deeper into hard-edge territory. The background reminds me of George Washington Lambert's 1905-10 works. The subject's face is one of the more softly-painted parts of the image.
A sketch or study done later than the others shown here.