Wendy and Marvin

Some readers may remember the Super Friends cartoon. In that cartoon were two tag-a-long kids Wendy and Marvin. While the cartoon revealed very little about them, the comic versions of them revealed much more about them.

Super Friends
Wendy and Marvin first appeared in the Super Friends cartoon and their back story was fleshed out in the comics published at the time. Wendy Harris and Marvin White were two kids that hanged around with the Justice League (the actual team isn't called the Super Friends) because Wendy is the niece of a friend of Batman and Marvin is the son of Diana Prince (a nurse who Wonder Women took her name from). The League tried to teach them how to be superheroes. Marvin and Wendy had a pet, Wonder Dog. Both Marvin and Wonder Dog were portrayed as intelligent, but bumbling. Because The All-New Super Friends Hour (the follow-up to the Super Friends) replaced the duo with the Wonder twin, the comics explained Marvin went to Ivy University (with the superhero Atom being a professor). Wendy went to Paraside Island and attended an Amazon university. However in the comics, they helped the Wonder Twins at least once.

DC Universe
In 2006, Wendy and Marvin were introduce into DC canon. The pair are twins (with the surname "Harris") and caretakers of Titans Towers. They helped repaired Cyborg after he was damaged. "Wonder Dog", a demonic monster in the form of a stray dog, killed Marvin and wounded Wendy. It is revealed the villain Calculator is their father and blamed the Titans for what happened to them. Kid Eternity summoned Marvin's soul, who tells Eternity to not work with the Titans since many of their members died. Later, the Calculator used alien technology to save Wendy, but it left her unable to use her legs. She befriended the similarly-disabled super heroine Oracle and eventually decided to act a junior version of Oracle called "Proxy".
Powers
In Super Friends, Wendy and Marvin have no powers. However, they had rely on resourcefulness and cleverness to make up for their lack of powers. Their pet, Wonder Dog, seemed to possesse human-level intelligence. He was capable of human speak and expressions.
In the DC universe, they are technical geniuses. Ravager claims they have "tenth-level geniuses" (a reference Brainiac and Brainiac 5 who are described as having twelfth-level intelligence). They were able to repair Cyborg and give him new capabilities.
References:
http://en.wikipedia.org/wiki/Wendy_and_Marvin