Kaijin 怪人 (Kaijin?), also known as Monster-Man or Phantom, is a common term heard throughout the Kamen Rider franchise. The only formal Kaijin in the Ultra Universe came to play in Ultraman vs. Kamen Rider. The Tsuburaya series, Triple Fighter is also known for it's Kaijin. This term is for sentient Kaiju who are human shape and human sized. Aliens/Seijin in the Ultra Universe, do not qualify as Kaijin, unless they cannot size change. In short, if it is, or can become a giant being, it is not a Kaijin.
Beings like the Victorians, who are physically humans, biologically humans, but granted a special power, not villains and come from Earth or not Seijin, as they are not 'monsters', they are people.