Role in the film
After having been abandoned by their old tribe for being too violent and too dim, they were chased into a canyon with their mother, believing themselves to be trapped by a monster.
When Uto found Tarzan, he instantly assumed that he was some sort of bird, and suggested throwing him off a cliff to make him fly. Kago turned down this idea, uninterested in something so small and weak, as well as pointing out Tarzan was not a bird, lacking feathers. However, both considered the idea that Tarzan was an ape absolutely ridiculous.
Later on, Uto caught Terk, and suggested throwing her off a cliff, but again Kago pointed out how stupid an idea this was, this time because Terk was a gorilla, as were they. When Mama Gunda finds out about Tarzan's tribe, she planned to overthrow Kerchak with her sons and take charge. They had to guard Terk from running away in the meantime, but she soon tricked them both and escaped.
Near the end of the movie, Uto caught one of the infant gorillas travelling with Kala, and suggested throwing him off the waterfall. Kala was angry and snarled, scaring him. Kago stood up for his brother, but in the scuffle, Kala and the infants were accidentally knocked onto a dangerous ledge, to both Kago and Uto's dismay.
Kago and Uto then had to catch Tarzan to keep him from telling the tribe that they planned to take over, however he proved far too small, quick-witted and agile for them to catch, and ended up giving them both quite a fright.
Uto is a dimwitted male who likes poking Kago and throwing things off cliffs to find out if they can fly and doesn't like to being called "Banana Brain".
Kago is a hulking and violent male who likes to smash rocks (among other things), and gets the hiccups when he gets angry and doesn't like to being poked.
Both brothers are extremely strong and have lots of stamina. Kago can punch rocks for hours without tiring, and Uto can take many blows without noticing. If Mama Gunda has sent them both up against Kerchak as she had planned, it is very likely that the two of them could have overpowered him.
Kago and Uto are constantly fighting, but nonetheless they are protective of each other - Kago gets angry when somebody scares Uto, and Uto gets angry when somebody deliberately provokes Kago. They are clearly not all bad though, since when Kala and the infant gorillas are almost knocked off the waterfall, both are quite dismayed, it clearly being an accident.