<Ul> <Li> Karen Gillan as Ruby Roundhouse, a commando, martial artist, and dance fighter who is Martha's avatar (after Martha was peer - pressured into playing with the others). Roundhouse is proficient in karate, t'ai chi, aikido and dance fighting and is reputed as "killer of men" because of both her skills and attractiveness . Similar to Bravestone, Roundhouse is the archetypically intelligent, athletic, beautiful, and scantily clad video game heroine of the story, with her only weakness being venom, such as that found in various animals and vegetation in the game . She is credited as "Martha". </Li> </Ul> <Li> Karen Gillan as Ruby Roundhouse, a commando, martial artist, and dance fighter who is Martha's avatar (after Martha was peer - pressured into playing with the others). Roundhouse is proficient in karate, t'ai chi, aikido and dance fighting and is reputed as "killer of men" because of both her skills and attractiveness . Similar to Bravestone, Roundhouse is the archetypically intelligent, athletic, beautiful, and scantily clad video game heroine of the story, with her only weakness being venom, such as that found in various animals and vegetation in the game . She is credited as "Martha". </Li> <Li> Mason Guccione as Alex Vreeke, a teenage boy who played, and was trapped inside, the Jumanji video game for 20 years . He is credited as "Young Alex". Colin Hanks portrays adult Alex Vreeke after he returns to 1996 and appears 20 years later, changing history as a result . Out of gratitude for Bethany saving his life, he named his eldest daughter after her . <Ul> <Li> Nick Jonas as Jefferson "Seaplane" McDonough, Alex Vreeke's avatar . McDonough is a mysterious guide, adventurer, and pilot in the game, who waits for and joins the other avatars as they attempt to escape Jumanji . McDonough's strengths are piloting and making margaritas that "Mouse" / "Fridge" takes a liking to . His only weakness is mosquito bites, which is bad for him and all the more ironic since the jungle, being a tropical one, is teeming with mosquitoes . He is credited as "Alex". During his twenty - year absence, only a few months have passed for him in the game world; and in the meantime, he lives in a treehouse built and once occupied by Alan Parrish (portrayed by Robin Williams and Adam Hann - Byrd in the 1995 film Jumanji). </Li> </Ul> </Li> <Ul> <Li> Nick Jonas as Jefferson "Seaplane" McDonough, Alex Vreeke's avatar . McDonough is a mysterious guide, adventurer, and pilot in the game, who waits for and joins the other avatars as they attempt to escape Jumanji . McDonough's strengths are piloting and making margaritas that "Mouse" / "Fridge" takes a liking to . His only weakness is mosquito bites, which is bad for him and all the more ironic since the jungle, being a tropical one, is teeming with mosquitoes . He is credited as "Alex". During his twenty - year absence, only a few months have passed for him in the game world; and in the meantime, he lives in a treehouse built and once occupied by Alan Parrish (portrayed by Robin Williams and Adam Hann - Byrd in the 1995 film Jumanji). </Li> </Ul>

Who played the grown up alex in jumanji