Kera, Kaira, Kairyō is the family name (surname).
Seijirō is the given/personal name. So in the US he would be Seijirō KERA. In Japanese custom, the surname goes first, and then the given name. There are no middle names in Japan. If you end up communicating with anyone in Japan, its probably best to NOT use the terms "first name" and "last name" as this always causes confusion. Best to use "surname/family name" and "given name".
There are only about 670 people with this family name, according to the site below, and I presume many of them are related somehow, so the chances of finding descendants is quite high. Note the diacritical marks in the name are not super important in trying to pinpoint this person. The kanji and their pronunciation are what's important. Even a rough or slightly incorrect pronunciation will not slow down your research. The
Obon Society mentioned above is one resource to check out. The Japanese embassy in your area may be another.
We had a member on the forum here not too long ago (within the last year, I think) who submitted a flag to the
Obon Society. The Obon Society explained that finding descendants is not easy, and that it may take a long time (i.e. years) to find the descendants. But in the end they were able to find the descendants within about three months, I think. In your case, you can also be optimistic that, with the right resources, you can probably find the descendants within the same time.