Lasombra was a ruthless treasure seeker, who stopped at nothing to get what he wants. He and his river pirate crew had no problem threatening a group of 10-year-olds with machetes if necessary. Lasombra himself is a step above his mooks, as he has no problem letting his own men be killed, is willing to let grade-school children die, and is quite resilient on his goals, even fighting off the effects of a deadly poison dart.
Lasombra was also quite intelligent, manipulative, and a quick thinker. He quickly gained Arnold's trust by posing as Eduardo, played off the real Eduardo's attempt to rescue the class as an attack by river pirates, and made the rest of the class turn on Arnold. He even predicted from the very beginning that Arnold would escape his lair, so he planted a tracker on the amulet he gave the boy.
Lasombra was a Central American man with dark brownish-grey hair, and with facial features and body proportions very similar to Eduardo's, allowing him to impersonate him easily. However, he seems to be several years older than Eduardo, as his face shows signs of age, and he has a bald spot.
During his theft of the Corazón over a decade ago, and in a picture on a newspaper in Arnold's room, Lasombra wore a black cloak over his upper body, and covered his face with a red cloth and a yellow skull mask. Lasombra most likely used this outfit during his thefts to avoid his face being recognized. It's also quite likely that the name "Lasombra" is derived from his appearance in this disguise. ("Lasombra" comes from "La Sombra", which is Spanish for "The Shadow".)
His regular attire is an aquamarine guayabera, pointed boots, dark blue pants, a white hat, and a machete on his back.
Lasombra was a river pirate, poacher and treasure hunter in the jungles of San Lorenzo. As seen in the episode "The Journal", he stole a sacred relic from the Green-Eyed People called the Corazón. However, Miles and Stella recovered it to pay back the Green-Eyes for previously saving their lives. He vowed to get the Corazón back if it was the last thing he ever did.
In Hey Arnold! The Jungle Movie, Lasombra sets up a fake contest for a trip to San Lorenzo and kidnaps Eduardo to later impersonate him with a fake mustache, all as a ruse to lure Arnold into his hideout in the jungle. When Arnold escapes captivity, Lasombra follows him into the Green-Eyed People's city and attempts to take the Corazón for himself, only to be foiled by the treasure's poison dart and fall to his death.
- Lasombra's voice actor in "The Journal" and the test animation for The Jungle Movie was never credited, causing some confusion as to who voiced him. A number of sources over the years claimed that Brian Doyle-Murray (who's best known voiceover work includes voicing The Flying Dutchman in Spongebob Squarepants and Captain K'nuckles in The Marvelous Misadventures of Flapjack) voiced him, however it is extremely unlikely, considering that he sounds completely different and only has one line. A more likely presumption on who voiced Lasombra is Carlos Alazraqui, who also voiced Eduardo, due to some recognizable inflections.
- In the original film's concept art, Lasombra is portrayed as a much older man. For the produced film, he was redesigned to resemble Eduardo more closely (and somewhat resemble his voice actor, Alfred Molina). His hair color is also a dark grey instead of white, making him look younger. The concept art shows him wearing the same outfit, though the colors are slightly different.
- In a leaked storyboard sequence from the original Jungle Movie, Lasombra is portrayed to be a character fluctuating between an approachable, yet volatile personality. He is seen trying to convince Arnold, unsuccessfully, to reveal the location of the Corazón. However, after saying he knows nothing of its whereabouts, Lasombra locks Arnold in a store cupboard and threatens to harm his friends.
- "I'll get the Corazon back, if it's the last thing I ever do!"