He is fairly young, and appears to be the only calf in the herd. Despite being the herd's youngest member (and the only one with a full head of hair), he still participates in the drills and has a fairly impressive knowledge of military terminology for one so young. In the first film, he is never referred to by name, with his parents referring to him as "Son."
Hathi, Jr. appears to be very friendly, as he becomes friends with Mowgli very quickly. He cares deeply for his friend's safety, such that he is able to convince his father to join in the search for Mowgli. He seems to idolize his father and desires to become a Colonel like him. He also appears to have a tendency to be left behind, as Hathi later notes that he has scolded Hathi, Jr. several times on staying with the herd.
Hathi, Jr. appears as part of the Jungle Patrol, which is lead by his father. The patrol is currently on a cross-country march, with periodic drills. During one such drill, he meets Mowgli. When Mowgli asks to join in the drill, Hathi, Jr. readily agrees. He instructs Mowgli to do as he does, but not to speak in ranks. During the drill, he informs Mowgli on military terminology and explains Colonel Hathi's various orders. He then participates in an inspection. He does well but is reminded by Hathi to keep his heels together.
After the rest of the herd leave to continue their drilling, Hathi, Jr. is initially left behind, partly because he was talking to Mowgli, and partly because Hathi forgot him. After being reminded by Winifred, Colonel Hathi returns to scold him, but the rest of the herd crashes into them. Hathi, Jr. remains unharmed and innocently tells his father that he forgot to tell the herd to halt.
He is seen marching with his herd when Bagheera stops the herd and asks for help to find Mowgli. Colonel Hathi refuses and is quickly reprimanded by Winifred. She asks how he would feel if their son was lost. Hathi claims that it would be a different matter, but Winifred notes that Mowgli is no different from Hathi, Jr. Then Hathi, Jr. pleads with his father to find Mowgli, saying that he is his friend and that if they don't help, Mowgli would be hurt. This results in his father forming several search parties. The herd then proceeds to search for Mowgli.
Jr. appears at the beginning where his father and the other elephants are stopping Baloo from going to the Man Village. Baloo escapes the elephants but sees Jr. looking at him. Baloo tells Jr. that he wants to see Mowgli. Jr. admits that he also misses Mowgli and points the way to the Man Village. Jr. is later seen stampeding with his father and the other elephants into a cave as man was in the jungle. After this scene, he is not seen for the rest of the film.
In the original novel, Hathi had three sons rather than one. It's heavily implied that the three were eventually merged into a single character.
Hathi Jr. is one of only two elephants in the Jungle Book franchise to have a mop of brown hair, for Winifred herself also used to sport a similar hairstyle as a child to her son as shown in Jungle Cubs.