Junior is very serious-minded and imperturbable, so he doesn't take kindly to trespassers, namely those who are not "cows," according to him. He talks in a deep, ominous voice and tries to maintain his composure when agitated. He also apparently has a crush on Mrs. Calloway, as he is seen complimenting her hat in a very coquettish and flirtatious manner.
Junior is Alameda Slim's gentlemanly "steed." Unusual for a villain's sidekick, he is portrayed as neither good nor bad, simply doing his job. He is very polite to cows and even flirts with Mrs. Calloway upon seeing her. He also tires of Buck's arrogance when the horse attempts to get through the mine's entrance, despite Junior saying repeatedly, "Cows only." Like Grace, Junior is immune to Slim's yodeling, but for Junior, it's because he's a buffalo, not a cow; whereas Grace (who is a cow) is just tone-deaf.
Later in the film, Maggie, Mrs. Calloway and Grace show up along with Lucky Jack (a jackrabbit who helped the cows reach Echo Mine), and Junior lets them all pass. When Buck questions Junior about this, the buffalo states, "They're cows," and when Buck brings up the rabbit, Junior replies, "Well, obviously, he was with the cows." Buck tries convincing Junior that he was with the cows too, but to no avail.
Sometime later, Buck tricks another horse into leaving his post so he could get back with Rico and then gloats. However, Buck is overheard by Junior who snorts angrily, as he had been right behind him. Junior is later seen chasing Buck around Echo Mine. He is shown to be immensely strong and survives a considerable fall after the cows inadvertently push him down the mine shaft. Junior soon makes his way out of the mine shaft and continues chasing Buck, but turns away after hearing Slim, who had been captured by the cows, calling for him.
At the end of the movie, Junior has left the services of Alameda Slim (since Slim is in jail) and arrives, along with Barry and Bob, at Little Patch of Heaven, where he courts Mrs. Calloway. He also dances with all the other farm animals as they celebrate Pearl's farm being saved and re-opened.
Junior is similar to Rico in that they both have deep, ominous voices and are completely obedient.
When Junior gets pushed and falls down the mine shaft, his yell is supplied by the classic Goofy holler sound effect.