The characters for "Reiuji" literally mean, in succession, "soul/spirit", "crow/raven", and "path". "Utsuho" means "emptiness/empty-hole/cave" in old Japanese. The character for Utsuho also means "sky", and can be pronounced "kuu". Hence her nickname, Okuu.
In ancient Chinese mythology, the three-legged crow is a mythical animal associated with the Sun, thought to be its guardian beast or, more often, the manifestation of the Sun's deity. This explains Utsuho's nuclear fusion power and association with heat.
This myth of the three-legged crow and the fact that the crow tengu Aya Shameimaru calls her "hell crow" twice may be why people refer to her as a crow, even though she's a raven. (Actually, most Japanese don't distinguish raven from crow, except zoologists and curious people. The fact that both raven and crow are from the corvus genus doesn't help.)
Utsuho appears to be the only final boss in a Windows game whose goal is explicitly conquering the world. A very common goal for villains in cartoons.
Utsuho is also the only final boss in a Windows game who is a subordinate of an earlier boss (Satori).
The beginning of Utsuho's theme, Solar Sect of Mystic Wisdom ~ Nuclear Fusion seems to be based on the theme of Yumemi Okazaki, Strawberry Crisis!!.
Utsuho's spell cards "Giga Flare" and "Mega Flare" share the name of the attacks used by the summon Bahamut of the Final Fantasy series.
Utsuho's unique danmaku is a extremely large, red glowing danmaku probably resembling a Red Giant or a massive bomb.
Utsuho's preferred attack patterns are a fast train of yellow danmaku that only follows one path and her spell cards often involves massive uses of her unique danmaku and changes its size on her will.
Utsuho is the only final boss in a Windows game that gives the player a warning before the spellcard begins. Two nuclear symbols appear, along with giant word "Caution" and Klaxons blaring in the background.
Utsuho's spellcard background in Subterranean Animism and Double Spoiler has a real-world picture of a cat on it. One reason ZUN might have included this in the background could be because of the wild cats that have colonized the abandoned city of Pripyat'. Pripyat' was in close proximity to Chernobyl, and after the catastrophe there the population was evacuated. Recent excursions back into the ghost town lead to the discovery of a small community of irradiated wild cats that live comfortably in the foundations of abandoned homes.
The red eye is proof that Utsuho obtained the power of fusion by absorbing the three-legged crow (Yatagarasu) because the three-legged crow's eyes are red, according to Chapter Three of Cage in Lunatic Runagate.
On the Interview of Caramel vol.8, ZUN mentioned that he tried to keep her from being indecent during the design process by adding a cannon on her arm instead of having an additional leg, thus solving the analogy of the third leg of the Yatagarasu.
Her right foot is supposed to be encased in concrete (representing the concrete sarcophagus that was built to contain the Chernobyl reactor), but is often depicted in fan art as a gray ruffled legwarmer or an armored greave. The electrons orbiting her other leg are also often omitted in fanart, and sometimes the control rod on her arm is too.
Her sprite animations in Unthinkable Natural Law greatly resemble and are most likely a homage to Souther of Hokuto no Ken fame, even having a spellcard attack similar to Souther's Fatal K.O., Nanto Hō-ōh Ken Ougi: Tenshou Juuji Hou. Because of this fans sometimes call that spellcard Nanto Hō-Ōkuu Ken Ougi: Tenshou Juuji Hou or Nanto Hō-Ōkuu Ken Ougi: Tenshou Reiuji Hou.
In Story of Eastern Wonderland's Stage 4, in the second part of the background, there was an orange-like shine that was shaped like the orb that Utsuho points to in her Subterranean Animism portrait.
An example of the above is her Geothermal "Nuclear Blaze Geyser" spell card, which highly resembles Terry's Trinity Geyser and Magneto's Magnetic Shockwave. Some other examples include her Hell Geyser skill card highly resembles Terry's Power Geyser and her Light Energy "High-Tension Blade" spell card, which resembles Terry's Buster Wolf.