Raku

Raku (楽) aka "Cammy" in the English version of the anime is the late cat that Yuzu Kurosaki once fed. Raku appears only in the anime, and doesn't appear in the manga.

Background
Before the series had officially started Yuzu Kurosaki had one day met Raku when it was just a young stray cat, out on the streets during a heavy rainy day. As it was nighttime Yuzu felt sorry for the yellow cat and gave it a sack of three biscuits to eat. Forming a sort of attachment to Yuzu, Raku tried to follow her home, but was hit and killed by a passing car. From then on, Raku's spirit haunted the Human World, as its attachment to Yuzu continued to bind it to the Human World.

Personality
According to Yuzu, the cat reminds her of their older brother Ichigo.

Appearance
Raku is a small cat with short blonde fur, and a white underbelly. Including long ears, and whiskers. Its eyes are bright green, almost emerald in color. It has the ability to transform into what seems like, a large Chinese lion guardian with a fiery yellow mane, and headband.

Karakura Lion Jet
Karakura Lion Jet (カラクラ・ライオン・ジェット), aka "Karakura Sonic Lion" in the English sub version): Transformed, Raku exhibits impressive speed, strength, and can surround itself with fire. Its power is close to a ranked Shinigami Officer as it is easily able to single-handedly overpower a Huge Hollow.

Soul Society arc
Raku returns sometime later as a spirit with unusually high Reiatsu, therefore attracting Hollows. When the Hollows endanger Yuzu, it transforms into a lion to protect her. Don Kanonji, who organizes the Karakura Superheroes in order to protect the area from Hollows, dubs it the "Karakura Lion Jet" upon first seeing it. He also claims it as the team's "hero mecha" (a common staple in Super Sentai styled anime), despite it not being mechanical.

After the Hollow is defeated, Raku having fulfilled its purpose, passes on. Nobody other than Yuzu knows that Raku is actually the "lion jet". Yuzu and her sister Karin Kurosaki compare Raku to their older brother, Ichigo Kurosaki.

Trivia

 * In both anime versions, Raku is named after a camel ("Raku" is japanese for "camel" and "Cammy" is short for camel). Because it is the same color as camelhair long johns, (Yuzu's "camel colored underwear" in the English version).
 * all information on Raku came from http://bleach.wikia.com/wiki/Raku