As many have pointed out, the Martian Manhunter is most famous for being a founding member of the Justice League, but he has a lot of other notable attributes and accomplishments. Get the skinny on Kara Zor-El's (Melissa Benoist) new ally below.
He's the Ostrander edition: Harewood told TVGuide.com that executive producer Andrew Kreisberg gave him the John Ostrander and Tom Mandrake Martian Manhunter comics to help him learn about the role, which gives a specific J'onn J'onzz (or John Jones) that inspires Harewood's character. Just in case you wanted to research, that's where you'd start.
Cat Grant could brand him Supermartian: Shapeshifting is just the beginning of J'onn's powers in the comic books. He also has super strength, speed and durability combined with regenerative healing. He also have invisibility, telepathy and telekinesis. Oh, he can fly too. He's basically the full house of superhero abilities.
He and Kara can also bond over dysfunctional family: The Ostrander and Mandrake series also says that J'onn has a brother named Ma'alefa'ak. That could be a surprise for Supergirl's J'onn who believes he's the last surviving member of his race. The only problem is, his comic book sibling wasn't the kind of guy you wanted to come to family reunions considering he's a sociopath.
There may be more Martians, just not green ones: J'onn J'onzz is perhaps the last of his race, but that doesn't mean there aren't other Martians. It's also a multicultural planet, you guys. According to comic book legend, green Martians don't mix well with the White Martians. In fact, they had a Martian Civil War which lead to J'onn's people banishing the White Martians to the "Still Zone" - which could potentially be the same as Supergirl's Phantom Zone.
He's got shapeshifter memory: According to Harewood's interview with Entertainment Weekly, a time will come soon where j'onn will shapeshift into Kara herself. There's no way he'll be able to keep up Supergirl appearances forever, which must mean that he can shift between the people he's already been. That's a useful skill to have in the bank.
J'onn likes it hot, but not too hot: You can't have all those awesome powers without a weakness, and the Martian Manhunter is unfortunately afraid of fire. It's better than being allergic to water when you're marooned on Earth, but you can bet J'onn isn't going to be taking anyone up on DEO barbecues any time soon.
More will unfold about J'onn and his specific Supergirl story when the show airs Mondays at 8/7c on CBS.
(Full disclosure: TVGuide.com is owned by CBS)