It centers around the travels and battles of one Aotsuki Ushio, who is constantly being stalked and aided by a gigantic, supernatural and sometimes invisible tiger (Some people describe it as akin to Calvin and Hobbes on acid).
 
Ushio's family maintains a Shinto temple in Japan, where 500 years ago, his samurai ancestor battled that same tiger to a standstill, and eventually trapped him against a rock using, a blessed spear called the "Beast Spear", which in addition to its intrinsic formidability as a weapon, can also boost the physical powers of the user. While cleaning out the temple storage, Ushio accidentally unseals the chamber which Tora was trapped in. Initially, Ushio wisely refused to remove the spear pinning Tora (とら, Japanese for tiger) down, as Tora unwisely makes the strategic blunder of mentioning that the first thing he would do upon being freed would be to devour Ushio; but Tora's unsealed presence attracts various demons to the premises, and Ushio is forced to free Tora in exchange for his help. The series focuses on their relationship, interspersed with battles against mythological foes, and with Tora's attempts to grapple with modern life and gobble up Ushio. 
Note: Won the Seiun Award for Best Comic of the Year in 1997. It also received the Shogakukan Manga Award for shounen in 1992.

Ushio thinks his father's tale of an ancient ancestor impaling a demon on a temple altar stone with the legendary Beast Spear is nuts, but when he finds the monster in his own basement, Ushio has to take another look at the family legend! Fortunately, Ushio knows it's best to let sleeping dogs lie and leave captured demons where they are. Unfortunately, the release of the monster's evil energies begins to beckon other demons to Ushio's hometown! To save his friends and family from the invading spirits, Ushio is forced to release Tora from his captivity. But will the cure prove to be worse than the curse? Will Ushio end his life a Tora-snack? Or will the Beast Spear keep Tora in line long enough to save the city? 
(Source: AniDB)

Long ago, a ferocious monster terrorized the land, until a samurai, wielding the legendary “Beast Spear”, sealed him away. 500 years later, a middle school student named Ushio Aotsuki accidently uncovers the monster in a hidden cellar under his family’s temple. The unsealed monster and the spear attract many other supernatural creatures to the temple and Ushio is forced to release the monster in order to defeat them. Ushio names the monster “Tora” and unwillingly work together to battle other spirits and demons. 
 
(Source: Crunchyroll)

The second season of Ushio to Tora (TV). 
 
Long ago, a ferocious monster terrorized the land, until a samurai, wielding the legendary “Beast Spear”, sealed him away. 500 years later, a middle school student named Ushio Aotsuki accidently uncovers the monster in a hidden cellar under his family’s temple. The unsealed monster and the spear attract many other supernatural creatures to the temple and Ushio is forced to release the monster in order to defeat them. Ushio names the monster “Tora” and unwillingly work together to battle other spirits and demons. 
 
(Source: Crunchyroll)

In this one shot volume Kazuhiro Fujita describes us the past of some characters of his masterpiece Ushio and Tora. In particular he describes the past of Tora, the tiger-monster that follows Ushio.

Ushio to Tora centers around the travails and battles of Ushio Aotsuki, who is constantly being stalked and aided by a gigantic, supernatural and sometimes invisible tiger-like monster. 
Ushio's family maintains a Shinto temple in Japan, where ~500 years ago, his samurai ancestor battled that same tiger to a standstill, and eventually trapped him against a rock using a blessed spear called the "Beast Spear," which in addition to its intrinsic formidability as a weapon, can also endow its wielder with the martial ability of the ancestor who originally wielded it. 
Ushio accidentally unsealed the cave Tora was trapped in. Initially, Ushio refuses to remove the spear pinning Tora down, but Tora's unsealed presence summons various monsters who wish to fight him, and by extension Ushio. Ushio is forced to free Tora, who would like to eat Ushio, but dares not since Ushio keeps the spear close by. The series focuses on their relationship, interspersed with battles against mythological foes, and with Tora's attempts to grapple with modern life. (Source: Wikipedia)

Tags